Fix race condition

Author: haslinghuis

src/js/serial_backend.js

@@ -150,7 +150,7 @@ export function initializeSerialBackend() {
                         finishClose(toggleStatus);
                     }
 
-                    mspHelper.setArmingEnabled(true, false, onFinishCallback);
+                    mspHelper?.setArmingEnabled(true, false, onFinishCallback);
                 }
             }
         }