The error message seems to indicate there is exception thrown when running the matcher. The error message gets swallowed because the describeMismatch assumes the matchState always contains the failure message, which may not be the case if the match does no finish successfully.

This pr add two things:

1. makes describeMismatch more robust to handle cases there matcher doesn't complete successfully.
2. make AndroidSemanticsNode to expect nullable fields which may be the reason matcher crashes.

The AndroidSemanticsNode.isHeader is only available above certain Android version, and it is likely the motog4 in devices lab is below the android version, which may be the reason the matcher crash in the first place.

The changes are in the second commit.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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