Example throws exception: ScrollController has no ScrollPosition attached

**Describe the bug**
When running the example application, the console displays the following exception:

```
======== Exception caught by scheduler library =====================================================
The following assertion was thrown during a scheduler callback:
The Scrollbar's ScrollController has no ScrollPosition attached.

A Scrollbar cannot be painted without a ScrollPosition.

The Scrollbar attempted to use the PrimaryScrollController. This ScrollController should be associated with the ScrollView that the Scrollbar is being applied to.
If a ScrollController has not been provided, the PrimaryScrollController is used by default on mobile platforms for ScrollViews with an Axis.vertical scroll direction.
To use the PrimaryScrollController explicitly, set ScrollView.primary to true on the Scrollable widget.

When the exception was thrown, this was the stack:
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 274:3 throw_
package:flutter/src/widgets/scrollbar.dart 1473:9
package:flutter/src/widgets/scrollbar.dart 1498:14 [_debugCheckHasValidScrollPosition]
package:flutter/src/widgets/scrollbar.dart 1429:14
package:flutter/src/scheduler/binding.dart 1434:7 [_invokeFrameCallback]
package:flutter/src/scheduler/binding.dart 1361:11 handleDrawFrame
package:flutter/src/scheduler/binding.dart 1057:9
lib/_engine/engine/frame_service.dart 145:9
dart-sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 47:11 internalCallback
====================================================================================================
```

I currently don't have the capacity to work on this myself, so any help is welcome! Thanks in advance!