ComboBoxes are empty when no datacontext

[carldebilly]

Bug Report

<ComboBox> won't work on Uno if there's no DataContext.

Current behavior

Not having a DataContext is causing the ComboBox to be empty when it's opened.

Expected behavior

Should behave the same, no matter if there's a DataContext or not.

Minimal reproduction of the problem with instructions

<StackPanel DataContext="{x:Null}">
	<ComboBox>
		<ComboBoxItem>A</ComboBoxItem>
		<ComboBoxItem>B</ComboBoxItem>
		<ComboBoxItem>C</ComboBoxItem>
		<ComboBoxItem>D</ComboBoxItem>
		<ComboBoxItem>E</ComboBoxItem>
		<ComboBoxItem>F</ComboBoxItem>
		<ComboBoxItem>G</ComboBoxItem>
		<ComboBoxItem>H</ComboBoxItem>
	</ComboBox>
</StackPanel>

Environment

Affected platform(s):
- [X] iOS
- [X] Android
- [X] WebAssembly
- [ ] Windows

[carldebilly]

Origin of the bug: unoplatform/Uno.Xaml-Controls-Gallery#4