[Glitch] dont crash with null-ref

Port 8568018935 to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/features/ui/index.js

@@ -174,8 +174,10 @@ class SwitchingColumnsArea extends React.PureComponent {
   }
 
   setRef = c => {
+    if (c) {
       this.node = c.getWrappedInstance();
     }
+  }
 
   render () {
     const { children, navbarUnder } = this.props;