Protect against null context in action invoke in release mode

[gspencergoog]

Description

Add some protection against a crash in release mode. The working theory at the moment is that it happens in some kind of race when removing widgets from the tree, where the primary focus context could be null.

Related Issues

• The method 'getElementForInheritedWidgetOfExactType' was called on null #64272

Tests

• Unable to make a regression test for this, since it only happens in release mode: any test would assert long before arriving at the point where this would crash.

[flutter-dashboard]

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[HansMuller]

Generally speaking, we don't make this kind of fix, since we don't really understand the cause and so we can't really produce a regression test. Perhaps the original issue should be left open (without the P3 priority) until we have sorted out the cause.

[gspencergoog]

Hans and I chatted, and decided that this is the best compromise at the moment. We're both uncomfortable with not having a regression test, but the (non-starter) alternative is to leave the bug in the code to get a better reproduction case. Since the code will already assert when this happens, we can find reproduction cases in debug builds just as usefully without impacting the customer.

I'm going to leave the bug open while we explore ways to get a reproduction case on a debug build (which will assert).