Fix focus issues with GUI

gui/src/advancedDynamicTexture.ts

@@ -102,12 +102,14 @@ module BABYLON.GUI {
                 return;
             }
 
-            if (!this._focusedControl) {
-                control.onFocus();
-            } else {
+            if (this._focusedControl) {
                 this._focusedControl.onBlur();
             }
 
+            if (control) {
+                control.onFocus();
+            }
+
             this._focusedControl = control;
         }