Check if XRSession exists before adding inputsourceschange listener (#5631)

Author: mrxz

src/systems/tracked-controls.js

@@ -17,6 +17,8 @@ module.exports.System = registerSystem('tracked-controls', {
   },
 
   onEnterVR: function () {
+    var xrSession = this.el.xrSession;
+    if (!xrSession) { return; }
     this.el.xrSession.addEventListener('inputsourceschange', this.onInputSourcesChange);
   },