📝 Provide detailed reproduction steps (if any)

While working on #6002 I noticed that the "should not throw an error when the destructor is not defined" unit case is causing DOM leaks.

Upon closer look it turned out that the promise returned by watchdog.create() is silently rejected (rejection caused by unhandled exception).

Two things here:

1. First this means that this test doesn't pass, as this call per unit test should not throw exception. If you adjust the test in a way that it waits for the promise resolution then it fails.
2. Why isn't this reported by Karma in a first place? I can see it reported when running Karma in debugging mode, but it is not reported to the console and it went unnoticed for a longer time.

📃 Other details

This issue might be related to #5897.

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