fix TypeError: Cannot read property ' __value' of undefined (#622)

Update mock to fix the type error: `TypeError: Cannot read property ' __value' of undefined`

Author: andrescalco

mock.js

@@ -23,7 +23,7 @@ const getValue = node => {
   if (typeof node === "number") {
     return node;
   }
-  return node[" __value"];
+  return node && node[" __value"];
 };
 
 class AnimatedValue {