Updating plugins can occasionally cause frontend resource loading to fail with HTTP 500 errors

Issue Summary

The request to webResources/1.0/resources REST resource can throw a NullPointerException and return with 500 error.

Steps to Reproduce

1. Run a standalone Bitbucket 7.8.0 in debug mode:

   mvn amps:debug-standalone -Dexec.args="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005" -Dproduct=bitbucket -Dproduct.version=7.8.0

2. Create a Pull Request under this project (or any other project): http://localhost:7990/bitbucket/projects/PROJECT_1/repos/rep_1/pull-requests/
3. In a new browser tab, go to the admin panel and navigate to UPM.
4. In UPM, install the `-new` plugin version (see attachments).
5. Open the PR page.
6. In UPM, install the `-another-one` plugin version (see attachments).
7. Refresh the PR page.

Expected Results

The page should render the PR details panel.

Actual Results

An HTTP 500 error is thrown by the /webResources/1.0/resources endpoint.

Workaround

Restart the instance after upgrading a plugin, uninstalling a plugin, or changing a plugin modules' enabled state.