And clear it in the widget layout store test, also fixing the tests
which didn't actually individually as they actually relied on the state
bleeding between tests.
* Support dynamic room predecessors in WidgetLayoutStore
* Improve TS correctness in WidgetLayoutStore
* Test to cover onNotReady to quieten SonarCloud
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Co-authored-by: Janne Mareike Koschinski <janne@kuschku.de>