Steps to reproduce were:
- Open an asset browser
- Open an asset library with assets in it
- Load a different file (e.g. File -> New -> General)
Didn't see a nice way to fix this with the current pre file load handler
callback we use for freeing asset libraries. Using this is cleaner, but
for now, the relationship between UI and asset system is too close
still, so better do explicit freeing at the right point in time.
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