fix this.parent is null

Author: jljsj33

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-drawer-menu",
-  "version": "0.5.6",
+  "version": "0.5.7",
   "description": "drawer menu animation component for react",
   "keywords": [
     "react",

src/Drawer.jsx

@@ -53,10 +53,10 @@ class Drawer extends React.PureComponent {
     };
   }
   componentDidMount() {
+    this.getParentAndLevelDom();
     if (this.props.parent) {
       this.container = this.defaultGetContainer();
     }
-    this.getParentAndLevelDom();
     this.forceUpdate();
   }