assert populated draft state in useDraftObservation()

src/frontend/hooks/draftObservation.ts

@@ -1,18 +1,34 @@
 import {useStore} from 'zustand';
 
 import {useDraftObservationContext} from '../contexts/DraftObservationContext';
-import {DraftState} from '../hooks/persistedState/usePersistedDraftObservationNew';
+import {
+  DraftState,
+  DraftStatePopulated,
+} from '../hooks/persistedState/usePersistedDraftObservationNew';
+import {useCallback} from 'react';
 
-function defaultSelector(state: DraftState) {
+function defaultSelector(state: DraftStatePopulated) {
   return state;
 }
 
-export function useDraftObservation<S = DraftState>(
-  selector: (state: DraftState) => S = defaultSelector as any,
+export function useDraftObservation<S = DraftStatePopulated>(
+  selector: (state: DraftStatePopulated) => S = defaultSelector as any,
 ) {
   const {store} = useDraftObservationContext();
-  const draftObservation = useStore(store, selector);
-  return draftObservation;
+
+  const assertPopulatedStateSelector = useCallback(
+    (state: DraftState) => {
+      if (!state.value) {
+        throw new Error('No observation to read');
+      }
+      return selector(state);
+    },
+    [selector],
+  );
+
+  const value = useStore(store, assertPopulatedStateSelector);
+
+  return value;
 }
 
 export function useDraftObservationActions() {