do not try to update state if it is not yet set

This happens when the backend pushes updates before the frontend has received and set the state when loading the page, first.

frontend/src/App.tsx

@@ -32,6 +32,9 @@ const reducer = (state: State, action: Action): State => {
     case 'SET_DATA':
       return action.data;
     case 'UPDATE_ATTRIBUTE': {
+      if (state === null) {
+        return null;
+      }
       return {
         ...state,
         value: setNestedValueByPath(state.value, action.fullAccessPath, action.newValue)