JSON parsing gives error message with Dash

Specific Domains Web Stack
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2

I figured it out. The error occurs because before uploading a file, stored_json_data is empty, and hence an empty tuple is returned, which cannot be parsed. The solution is to test if isempty(stored_json_data), and if true I just return a line break.

callback!(app,
          Output("select_targets", "children"), 
          Input("store_data", "data"),
          prevent_initial_call=true) do stored_json_data

              if isempty(stored_json_data)
                  return html_br()
              else 
                  
                  # convert from JSON to dataframe
                  d = DataFrame(JSON.parse(stored_json_data)) # error is here
                  
                  return html_div([
                      html_br(), 
                      html_p("Select target"),
                      dcc_dropdown(id="target",
                                   options=[(label = i, value=i) for i in unique(d.group)],
                                   style=Dict("width" => "40%")),
                      html_button("Create graph", id="button")
                  ])
              end
          end