ltk: NPE in TextFileChange.acquireDocument

[jukzi]

i don't know how to reproduce, but obviously TextFileChange.fAcquireCount is not Multithreading safe, maybe fAcquireCount could even be totally eliminated by just using asking fBuffer!=null instead

eclipse.buildId=4.31.0.I20240204-1800 java.version=21.0.2

java.lang.NullPointerException: Cannot invoke "org.eclipse.core.filebuffers.ITextFileBuffer.getDocument()" because "this.fBuffer" is null
	at org.eclipse.ltk.core.refactoring.TextFileChange.acquireDocument(TextFileChange.java:211)
	at org.eclipse.jdt.core.refactoring.CompilationUnitChange.acquireDocument(CompilationUnitChange.java:85)
	at org.eclipse.ltk.core.refactoring.TextChange.getCurrentDocument(TextChange.java:310)
	at org.eclipse.ltk.core.refactoring.TextChange.getPreviewDocument(TextChange.java:517)
	at org.eclipse.ltk.core.refactoring.TextChange.getPreviewDocument(TextChange.java:398)
	at org.eclipse.jdt.core.manipulation.CUCorrectionProposalCore.getAdditionalProposalInfo(CUCorrectionProposalCore.java:135)
	at org.eclipse.jdt.internal.ui.text.correction.proposals.RefactoringCorrectionProposalCore.getAdditionalProposalInfo(RefactoringCorrectionProposalCore.java:89)
	at org.eclipse.jdt.ui.text.java.correction.CUCorrectionProposal.getAdditionalProposalInfo(CUCorrectionProposal.java:191)
	at org.eclipse.jface.text.contentassist.AdditionalInfoController$Timer$2$1.run(AdditionalInfoController.java:116)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)