[Bug] Shell: First navigation call on background thread throws the exception "Can only be called on ui thread!"

[MaxFmi]

The first navigation call end up in this else case:

Xamarin.Forms/Xamarin.Forms.Core/Shell/ShellNavigationManager.cs

Line 166 in ae8e52f

await _shell.CurrentItem.CurrentItem.GoToAsync(navigationRequest, queryData, animate, isRelativePopping);

Which is not surrounded by "InvokeOnMainThread" and throws the exception "Can only be called on ui thread!".


The navigation calls above are wrapped by "InvokeOnMainThread" and work as expected:

Xamarin.Forms/Xamarin.Forms.Core/Shell/ShellNavigationManager.cs

Line 160 in ae8e52f

return _shell.CurrentItem.CurrentItem.GoToAsync(navigationRequest, queryData, animate, isRelativePopping);

Description

Steps to Reproduce

1. Set MainPage of your Shell application
2. Invoke navigation through "await Shell.Current.GoToAsync(route);"
3. We end up in line 166 of ShellNavigationManager

Expected Behavior

Should navigate as expected.

Actual Behavior

Exception get thrown: "Can only be called on ui thread!"

Basic Information

• Version with issue: 4.8.0.1687 (but is still the case in 5.x -> see links above)
• Last known good version: -
• Platform Target Frameworks:
  • iOS: 13.x and 14.x
  • Android: not tested but we expect that it behaves the same way as on iOS

Workaround

Wrap "await Shell.Current.GoToAsync(route);" in "await Device.InvokeOnMainThreadAsync(...);"

[jfversluis]

Reverted the PR for this as it caused an NRE in another scenario. At least this bug has a workaround. We should see if we can fix this properly.

[jfversluis]

@AlleSchonWeg

#14730 (comment)

Moving the discussion to here :)

Not sure if that's the issue. The actual exception happened on this line

Xamarin.Forms/Xamarin.Forms.Core/Shell/ShellNavigationManager.cs

Line 253 in 7958ff9

element.SetValue(ShellContent.QueryAttributesProperty, MergeData(element, query, isPopping));

Where element would be null