Fix custom component registry access #6811
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PR Description
There is a small window where the loader is reloading (a new config is being applied) while a custom component is concurrently being re-evaluated.
To get its config, the custom component must check the custom component registry during its evaluation. This registry is re-created every time a config is applied. This means that there is a possibility that the custom component being re-evaluated is looking at a registry that is not fully initialized and might not have yet loaded the config needed. This results in a panic.
The provided fix locks the mutex before the re-evaluation to prevent both evaluations from running concurrently. In other words, the re-evaluation is paused when a new config is applied.
I added a test that triggers the bug 80% of the time on my computer. With the fix it always passes.
I also ran all import tests 50 times to check for regressions.
Which issue(s) this PR fixes
Fixes #6795
PR Checklist