Fix incorrect kIsWeb usage instead of withData

[divan]

I was using this widget with image preview with data bytes being available on MacOS platform. There is a field called withData to signal widget that bytes are available. Default value equals to kIsWeb.

Somehow instead of using withData on

form_builder_file_picker/lib/src/form_builder_file_picker.dart

Line 246 in 7311405

? kIsWeb

, kIsWeb was used. So on MacOS platform this was trying to read file from path (which is null) instead of bytes. That resulted in a red error box and stacktrace:

Another exception was thrown: Null check operator used on a null value
Another exception was thrown: A RenderFlex overflowed by 99827 pixels on the bottom.


══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following _TypeError was thrown building LayoutBuilder:
Null check operator used on a null value

The relevant error-causing widget was:
  LayoutBuilder
  LayoutBuilder:file:///Users/divan/.pub-cache/hosted/pub.dev/form_builder_file_picker-4.0.0/lib/src/form_builder_file_picker.dart:220:12

When the exception was thrown, this was the stack:
#0      _FormBuilderFilePickerState.defaultFileViewer.<anonymous closure>.<anonymous closure> (package:form_builder_file_picker/src/form_builder_file_picker.dart:249:66)
#1      new _GrowableList.generate (dart:core-patch/growable_array.dart:136:28)
#2      _FormBuilderFilePickerState.defaultFileViewer.<anonymous closure> (package:form_builder_file_picker/src/form_builder_file_picker.dart:231:26)