fix(menu): remove exception when async loading

Fixes #4687.

Author: ThomasBurleson

src/components/menu/js/menuServiceProvider.js

@@ -410,24 +410,22 @@ function MenuProvider($$interimElementProvider) {
         right: boundryNodeRect.right - MENU_EDGE_MARGIN
       };
 
-      var alignTarget, alignTargetRect, existingOffsets;
+      var alignTarget, alignTargetRect = { top:0, left : 0, right:0, bottom:0 }, existingOffsets  = { top:0, left : 0, right:0, bottom:0  };
       var positionMode = opts.mdMenuCtrl.positionMode();
 
       if (positionMode.top == 'target' || positionMode.left == 'target' || positionMode.left == 'target-right') {
-        // TODO: Allow centering on an arbitrary node, for now center on first menu-item's child
         alignTarget = firstVisibleChild();
-        if (!alignTarget) {
-          throw Error('Error positioning menu. No visible children.');
+        if ( alignTarget ) {
+          // TODO: Allow centering on an arbitrary node, for now center on first menu-item's child
+          alignTarget = alignTarget.firstElementChild || alignTarget;
+          alignTarget = alignTarget.querySelector('[md-menu-align-target]') || alignTarget;
+          alignTargetRect = alignTarget.getBoundingClientRect();
+
+          existingOffsets = {
+            top: parseFloat(containerNode.style.top || 0),
+            left: parseFloat(containerNode.style.left || 0)
+          };
         }
-
-        alignTarget = alignTarget.firstElementChild || alignTarget;
-        alignTarget = alignTarget.querySelector('[md-menu-align-target]') || alignTarget;
-        alignTargetRect = alignTarget.getBoundingClientRect();
-
-        existingOffsets = {
-          top: parseFloat(containerNode.style.top || 0),
-          left: parseFloat(containerNode.style.left || 0)
-        };
       }
 
       var position = {};