FIR IDE: fix flaky completion test

idea/idea-fir/src/org/jetbrains/kotlin/idea/completion/contributors/helpers/completionHelpers.kt

@@ -17,9 +17,11 @@ internal fun InsertionContext.addSymbolAndInvokeCompletion(symbol: String) {
 
 private fun invokeCompletion(context: InsertionContext) {
     ApplicationManager.getApplication().invokeLater {
+        if (!context.editor.isDisposed) {
             CodeCompletionHandlerBase(CompletionType.BASIC, true, false, true)
                 .invokeCompletion(context.project, context.editor)
         }
+    }
 }
 
 private fun addSymbolToCompletion(context: InsertionContext, symbol: String) {