Using content relationship field (inside slice)

Product Custom Type

{
  "Main" : {
    "uid" : {
      "type" : "UID",
      "config" : {
        "label" : "UID"
      }
    },
    "product_title" : {
      "type" : "StructuredText",
      "config" : {
        "single" : "heading1",
        "label" : "Product title"
      }
    },
    "product_code" : {
      "type" : "Text",
      "config" : {
        "label" : "Product code",
        "placeholder" : "Product code"
      }
    },
    "category" : {
      "type" : "Link",
      "config" : {
        "select" : "document",
        "customtypes" : [ "category" ],
        "label" : "Category",
        "placeholder" : "Product category"
      }
    },
    "subcategory" : {
      "type" : "Link",
      "config" : {
        "select" : "document",
        "customtypes" : [ "category" ],
        "label" : "Subcategory",
        "placeholder" : "Product subcategpry"
      }
    },
    "price" : {
      "type" : "Group",
      "config" : {
        "fields" : {
          "value" : {
            "type" : "Number",
            "config" : {
              "label" : "Value",
              "placeholder" : "Product price value"
            }
          },
          "currency" : {
            "type" : "Select",
            "config" : {
              "options" : [ "HRK", "EUR" ],
              "default_value" : "HRK",
              "label" : "Currency",
              "placeholder" : "Product price currency"
            }
          }
        },
        "label" : "Price"
      }
    },
    "disclaimer" : {
      "type" : "StructuredText",
      "config" : {
        "multi" : "paragraph,strong,em,rtl",
        "allowTargetBlank" : true,
        "label" : "Disclaimer",
        "placeholder" : "Pricing disclaimer"
      }
    },
    "pricing_date" : {
      "type" : "Date",
      "config" : {
        "label" : "Pricing date",
        "placeholder" : "Date from which the current price is valid"
      }
    },
    "body" : {
      "type" : "Slices",
      "fieldset" : "Slice zone",
      "config" : {
        "labels" : null,
        "choices" : {
          "technical_specifications" : {
            "type" : "Slice",
            "fieldset" : "TechnicalSpecifications",
            "description" : "Product's technical specifications",
            "icon" : "list",
            "display" : "list",
            "non-repeat" : {
              "title" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading2,heading3",
                  "label" : "Title",
                  "placeholder" : "Section title"
                }
              }
            },
            "repeat" : {
              "name" : {
                "type" : "Text",
                "config" : {
                  "label" : "Name",
                  "placeholder" : "Technical specification name"
                }
              },
              "value" : {
                "type" : "Text",
                "config" : {
                  "label" : "Value",
                  "placeholder" : "Technical specification value"
                }
              }
            }
          },
          "related_products" : {
            "type" : "Slice",
            "fieldset" : "RelatedProducts",
            "description" : "Related products",
            "icon" : "add_circle",
            "display" : "list",
            "non-repeat" : {
              "title" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading2,heading3",
                  "label" : "Title",
                  "placeholder" : "Section title"
                }
              }
            },
            "repeat" : {
              "products" : {
                "type" : "Link",
                "config" : {
                  "select" : "document",
                  "customtypes" : [ "product" ],
                  "label" : "Products",
                  "placeholder" : "Related products"
                }
              }
            }
          },
          "details_list" : {
            "type" : "Slice",
            "fieldset" : "DetailsList",
            "description" : "List of product details",
            "icon" : "format_list_bulleted",
            "display" : "list",
            "non-repeat" : {
              "title" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading2,heading3",
                  "label" : "Title",
                  "placeholder" : "Section title"
                }
              }
            },
            "repeat" : {
              "details" : {
                "type" : "StructuredText",
                "config" : {
                  "multi" : "paragraph,strong,em,list-item,o-list-item,rtl",
                  "allowTargetBlank" : true,
                  "label" : "Details",
                  "placeholder" : "Product details"
                }
              }
            }
          },
          "characteristics" : {
            "type" : "Slice",
            "fieldset" : "Characteristics",
            "description" : "Product characteristics",
            "icon" : "edit",
            "display" : "list",
            "non-repeat" : {
              "title" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading2,heading3",
                  "label" : "Title",
                  "placeholder" : "Section title"
                }
              }
            },
            "repeat" : {
              "heading" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading3,heading4,heading5,heading6",
                  "label" : "Heading",
                  "placeholder" : "Characteristic's heading"
                }
              },
              "description" : {
                "type" : "StructuredText",
                "config" : {
                  "multi" : "paragraph,preformatted,heading1,heading2,heading3,heading4,heading5,heading6,strong,em,hyperlink,image,embed,list-item,o-list-item,rtl",
                  "allowTargetBlank" : true,
                  "label" : "Description",
                  "placeholder" : "Characteristic's description"
                }
              }
            }
          },
          "overview" : {
            "type" : "Slice",
            "fieldset" : "Overview",
            "description" : "Product overview",
            "icon" : "accessibility",
            "display" : "list",
            "non-repeat" : {
              "title" : {
                "type" : "StructuredText",
                "config" : {
                  "single" : "heading2,heading3",
                  "label" : "Title",
                  "placeholder" : "Section title"
                }
              },
              "description" : {
                "type" : "StructuredText",
                "config" : {
                  "multi" : "paragraph,preformatted,heading1,heading2,heading3,heading4,heading5,heading6,strong,em,hyperlink,image,embed,list-item,o-list-item,rtl",
                  "allowTargetBlank" : true,
                  "label" : "Description",
                  "placeholder" : "Product summary"
                }
              }
            },
            "repeat" : {
              "image" : {
                "type" : "Image",
                "config" : {
                  "constraint" : { },
                  "thumbnails" : [ ],
                  "label" : "Image"
                }
              },
              "alt_text" : {
                "type" : "Text",
                "config" : {
                  "label" : "Alt text",
                  "placeholder" : "Alternative text"
                }
              }
            }
          }
        }
      }
    }
  }
}

For the GraphQuery I tried something like this query: Content relationship fields inside a group - #3 by Levi
When using this this I just get the same response. I tried modifying the query in many ways to get all the data but I get the error that 'data' is not defined.
I just need a little help to understand how exactly this should work.