Error in Java Model (code 980): JDT/Autocomplete wrecked sometimes

I'm using the latest Eclipse SDK and noticed that from time to time autocompletion is completely wrecked, from this point on I don't get any or only incomplete proposals.

I found the following exception in the log:

```
Java Model Exception: Error in Java Model (code 980): Index out of bounds
at org.eclipse.jdt.internal.core.Openable.codeComplete(Openable.java:126)
at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:367)
at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:357)
at org.eclipse.jdt.ui.text.java.JavaContentAssistInvocationContext.computeKeywordsAndContext(JavaContentAssistInvocationContext.java:307)
at org.eclipse.jdt.ui.text.java.JavaContentAssistInvocationContext.getCoreContext(JavaContentAssistInvocationContext.java:179)
at org.eclipse.jdt.internal.ui.text.java.SWTTemplateCompletionProposalComputer.computeCompletionEngine(SWTTemplateCompletionProposalComputer.java:161)
at org.eclipse.jdt.internal.ui.text.java.AbstractTemplateCompletionProposalComputer.computeCompletionProposals(AbstractTemplateCompletionProposalComputer.java:71)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:348)
at org.eclipse.jdt.internal.ui.text.java.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:340)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.collectProposals(ContentAssistProcessor.java:333)
at org.eclipse.jdt.internal.ui.text.java.ContentAssistProcessor.computeCompletionProposals(ContentAssistProcessor.java:289)
at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$10(AsyncCompletionProposalPopup.java:378)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
at org.eclipse.jface.text.contentassist.AsyncCompletionProposalPopup.lambda$9(AsyncCompletionProposalPopup.java:377)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768)
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760)
at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)

```

Even worse I noticed that editing Java files stopped to function after a while and even saving files no longer works...