anchor.diff:        });
       }
       if (widget.onFileSelected != null) {
  -      widget.onFileSelected(file);
  -    }
  -  }
  -}
  -
  -Future<DevToolsJsonFile> importFileFromPicker({
  -  @required List<String> acceptedTypes,
  +      widget.onFileSelected!(file);
  +    }
  +  }
  +}
  +
  +Future<DevToolsJsonFile?> importFileFromPicker({
  +  required List<String> acceptedTypes,
   }) async {
     final acceptedTypeGroups = [XTypeGroup(extensions: acceptedTypes)];
     final file = await openFile(acceptedTypeGroups: acceptedTypeGroups);
  
positive.code:           children: [
              ElevatedButton(
                onPressed: importedFile != null
                    ? () => widget.onAction(importedFile)
                    : null,
                child: MaterialIconLabel(
                  label: widget.actionText,
  
negative.code: 
  class DualFileImportContainer extends StatefulWidget {
    const DualFileImportContainer({
      required this.firstFileTitle,
      required this.secondFileTitle,
      required this.firstInstructions,
      required this.secondInstructions,
      required this.actionText,
      required this.onAction,
      Key? key,
    });
  
    final String firstFileTitle;
  