Fix bug for shutdown button in runsession

Fix bug:Kernel shutdown button in Runsessions doesn't work when
terminal is unavaliable.

packages/running/src/index.ts

@@ -321,6 +321,9 @@ class RunningSessions extends Widget {
       return;
     }
 
+    // Create a dummy div if terminals are not available.
+    shutdownTerms = shutdownTerms || document.createElement('div');
+
     // Check for terminals shutdown.
     if (ElementExt.hitTest(shutdownTerms, clientX, clientY)) {
       showDialog({