NullPointerException in onDidCompletionItemSelect

[mfussenegger]

Using latest master of eclipse.jdt.ls (45f31c1)

As far as I can tell the error only happens when using some snippets:

java.lang.NullPointerException: Cannot invoke "java.util.List.size()" because the return value of "org.eclipse.jdt.ls.core.internal.handlers.CompletionResponse.getItems()" is null
	at org.eclipse.jdt.ls.core.internal.handlers.CompletionHandler.onDidCompletionItemSelect(CompletionHandler.java:137)
	at org.eclipse.jdt.ls.core.internal.JDTDelegateCommandHandler.executeCommand(JDTDelegateCommandHandler.java:152)
	at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler$1.run(WorkspaceExecuteCommandHandler.java:230)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:45)
	at org.eclipse.jdt.ls.core.internal.handlers.WorkspaceExecuteCommandHandler.executeCommand(WorkspaceExecuteCommandHandler.java:220)
	at org.eclipse.jdt.ls.core.internal.handlers.JDTLanguageServer.lambda$4(JDTLanguageServer.java:561)
	at org.eclipse.jdt.ls.core.internal.BaseJDTLanguageServer.lambda$0(BaseJDTLanguageServer.java:79)
	at java.base/java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:646)
	at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
	at java.base/java.util.concurrent.ForkJoinPool$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)

I can produce this if I write sysout and then trigger completion. The response from the server is:

 {
    result = {
      isIncomplete = false,
      items = { {
          command = {
            arguments = { "1", "0" },
            command = "java.completion.onDidSelect",
            title = ""
          },
          data = {
            COMPLETION_EXECUTION_TIME = "286",
            pid = "0",
            rid = "1",
            uri = "file:///path/to/Example.java"
          },
          detail = "print to standard out",
          insertText = "System.out.println(${0});",
          insertTextFormat = 2,
          kind = 15,
          label = "sysout"
        } }
    }
  } }

If the client then triggers the java.completion.onDidSelect command the exception is raised in:

https://github.com/eclipse/eclipse.jdt.ls/blob/45f31c1f1a940724491d32ced30480e80603d02b/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/CompletionHandler.java#L137

From what I can tell there are two different CompletionResponse instances in the CompletionResponses cache.

One is set here: https://github.com/eclipse/eclipse.jdt.ls/blob/45f31c1f1a940724491d32ced30480e80603d02b/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/contentassist/SnippetCompletionProposal.java#L324-L325
This one has no items.

The other one here: https://github.com/eclipse/eclipse.jdt.ls/blob/45f31c1f1a940724491d32ced30480e80603d02b/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/contentassist/CompletionProposalRequestor.java#L255
This one would have items, but is not retrieved in the onDidSelect

[testforstephen]

@jdneo for a look.