diff --git a/openlp/core/ui/mainwindow.py b/openlp/core/ui/mainwindow.py
index 8f3a8aa0f..84625e51f 100644
--- a/openlp/core/ui/mainwindow.py
+++ b/openlp/core/ui/mainwindow.py
@@ -552,7 +552,10 @@ class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow, LogMixin, RegistryPropert
         wait_dialog.setAutoClose(False)
         wait_dialog.setCancelButton(None)
         wait_dialog.show()
-        for thread_name in self.application.worker_threads.keys():
+        thread_names = self.application.worker_threads.keys()
+        for thread_name in thread_names:
+            if thread_name not in self.application.worker_threads.keys():
+                continue
             self.log_debug('Waiting for thread %s' % thread_name)
             self.application.processEvents()
             thread = self.application.worker_threads[thread_name]['thread']