dont crash with null-ref (#12274)

app/javascript/mastodon/features/ui/index.js

@@ -164,7 +164,9 @@ class SwitchingColumnsArea extends React.PureComponent {
   }
 
   setRef = c => {
-    this.node = c.getWrappedInstance();
+    if (c) {
+      this.node = c.getWrappedInstance();
+    }
   }
 
   render () {