Skip updates to missing components

This fixes an issue that arose in deea1f, where we started flushing pending updates to the canvas editor when unmounting it. Deleting a component removes it from the application state and also unmounts the canvas editor, causing an update to a non-existent component and resulting in an invalid application state. We discussed several different places in which to check for the existence of the data, and this choice is debatable, but it's the simplest; the commit also adds a note to indicate that this might require revisiting.

packages/builder/src/reducers/components.js

@@ -206,12 +206,24 @@ export default (state=defaultState, action) => {
       }
 
     case 'UPDATE_COMPONENT':
-      return {
-        ...state,
-        [action.id]: {
-          ...state[action.id],
-          ...action.data,
-        },
+      if (state[action.id]) {
+        return {
+          ...state,
+          [action.id]: {
+            ...state[action.id],
+            ...action.data,
+          },
+        }
+      } else {
+        console.log(`Skipping update to missing component ${ action.id }`)
+        // TODO: This fallback is designed to catch a very specific bug
+        // that occured when flushing pending updates to a deleted component.
+        // In principle, the same issue is present with all other reducers,
+        // although it is extremely unlikely to occur.
+        // In the long run, it may be worthwhile moving this check into
+        // component logic, or, alternatively, making it universal to
+        // all actions (e.g. by checking at the top of this reducer).
+        return state
       }
 
     case 'ADD_FILES':