Fixed issue with simple object as binding context.

apps/tests/ui/bindable-tests.ts

@@ -1220,4 +1220,15 @@ export function test_BindingToRelatedProps() {
     model.prop2 = "Tralala";
     
     TKUnit.assertEqual(target2.get('targetProp2'), "Tralala");
+}
+
+export function test_only_Bindable_BindingContext_Null_DoesNotThrow() {
+        var options: bindable.BindingOptions = {
+            sourceProperty: "a.b",
+            targetProperty: "test"
+        }
+        var obj = new bindable.Bindable();
+        obj.bind(options);
+        obj.bindingContext = new observable.Observable({ a: "b" });
+        obj.bindingContext = null;
 }