Add null check for StorageAccess interface in storageAccessFromPath
In slotInterfacesRemoved we do a remove+add cycle
During that cycle the backend object is null, so querying the StorageAccess interface will return null.
This means that when something calls storageAccessFromPath in reaction to the removed signal we will get a null StorageAccess.
The overall design is a bit shaky here, but we can handle it more gracefully by checking the result for null
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