Fix import of entire delivery.xml

[AnNeufang]

Is there an existing issue for this?

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Current Behavior

The import of the entire content of a delivery.xml file (for example placed inside a third-party delivery) with the import strategy "Import Subtree" leads to an "Import Subtree: null java.lang.NullPointerException: null" and also shows up "Invalid thread access" message.

Expected Behavior

Both ways of importing delivery file (entire component strucutre of the given root component from file or "only" a single component) should succeed.

Steps To Reproduce

1. Add a new component
2. Setup "Import Subtree" als import Strategy in "Import: Version" or "Import: Git Commits" tab and give the path to an multi-component delivery.xml file and the name of the root component that should be imported to the parent documentation.
3. Run "Test" on the new component or create a new delivery.
4. Check Status Windows output or log file.