Reproducible by Aut Tests where:
org.netbeans.editor.BaseDocument.insertString(...) is called outside AWT-EventQueue, see:
Created attachment 43968 [details]
Sorry, but I do not see how is this related to AnnotationHolder - it is nowhere in the thread dump.
Created attachment 43972 [details]
Correct Thread Dump
Sorry, I attached wrong TD. Please see the Correct Thread Dump.
Ok, thanks. I will try to fix for M10.
Also reported as issue #107402.
*** This issue has been marked as a duplicate of 107402 ***