fix(proxy-state-tree): ensure prototype before getting descriptor

Author: christianalfoni

packages/node_modules/proxy-state-tree/src/Proxyfier.ts

@@ -286,7 +286,8 @@ export class Proxifier {
         if (typeof prop === 'symbol' || prop in Object.prototype || target[prop] instanceof Date)
           return target[prop]
 
-        const descriptor = Object.getOwnPropertyDescriptor(target, prop) || Object.getOwnPropertyDescriptor(Object.getPrototypeOf(target), prop)
+        
+        const descriptor = Object.getOwnPropertyDescriptor(target, prop) || (Object.getPrototypeOf(target) && Object.getOwnPropertyDescriptor(Object.getPrototypeOf(target), prop))
 
         if (descriptor && 'get' in descriptor) {
           const value = descriptor.get.call(proxy)