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;
 }

ui/core/bindable.ts

@@ -192,10 +192,8 @@ export class Binding {
             this.sourceOptions = undefined;
         }
     }
-
+    
-    public bind(source: Object): void {
+    private sourceAsObject(source: any): any {
-        this.clearSource();
-
         /* tslint:disable */
         let objectType = typeof source;
         if (objectType === "number") {
@@ -208,6 +206,13 @@ export class Binding {
             source = new String(source);
         }
         /* tslint:enable */
+        return source;
+    }
+
+    public bind(source: Object): void {
+        this.clearSource();
+
+        source = this.sourceAsObject(source);
 
         if (!types.isNullOrUndefined(source)) {
             this.source = new WeakRef(source);
@@ -559,7 +564,7 @@ export class Binding {
             let resolvedObj = objectsAndProperties[objectsAndProperties.length - 1].instance;
             let prop = objectsAndProperties[objectsAndProperties.length - 1].property;
             return {
-                instance: new WeakRef(resolvedObj),
+                instance: new WeakRef(this.sourceAsObject(resolvedObj)),
                 property: prop
             }
         }