fix(dialog): guard against missing focus traps upon removal.

Author: jelbourn

src/components/dialog/dialog.js

@@ -624,8 +624,13 @@ function MdDialogProvider($$interimElementProvider) {
       options.hideBackdrop(options.$destroy);
 
       // Remove the focus traps that we added earlier for keeping focus within the dialog.
-      topFocusTrap.parentNode.removeChild(topFocusTrap);
-      bottomFocusTrap.parentNode.removeChild(bottomFocusTrap);
+      if (topFocusTrap && topFocusTrap.parentNode) {
+        topFocusTrap.parentNode.removeChild(topFocusTrap);
+      }
+
+      if (bottomFocusTrap && bottomFocusTrap.parentNode) {
+        bottomFocusTrap.parentNode.removeChild(bottomFocusTrap);
+      }
 
       // For navigation $destroy events, do a quick, non-animated removal,
       // but for normal closes (from clicks, etc) animate the removal