Error handling enhancements

[ndonkoHenri]

Closes #3149

Tasks:

• Py end: added assertion checks + little code refactoring (typing) + made some props "required"
• Dart end: make error messages clearer + added isVisible filter when getting children

Breaking changes:

The following are now "required"

• AnimatedSwitcher.content
• Banner.content, Banner.actions
• BottomSheet.content
• CupertinoActionSheetAction.content
• DataRow.cells
• DataTable.columns
• DrageTarget.content
• Draggable.content
• ExpansionTile.title
• MenuBar.controls
• Pagelet.content
• RadioGroup.content
• SafeArea.content
• ShaderMask.shader
• WindowDragArea.content

[FeodorFitsner]

🤯🤯🤯 Looking...

[FeodorFitsner]

I honestly don't think those now-required properties are "breaking". Without them controls don't have any sense, right?

[ndonkoHenri]

I honestly don't think those now-required properties are "breaking". Without them controls don't have any sense, right?

I considered as breaking in the sense that "any code without those properties will break". Ex: I had to modify some of the test files because they were some tests were "failing" as a result of those required-parameters being missing.