fix: replace throw with debug warning to fix breaking change (#3206)

* fix: replace throw with debug warning to fix breaking change

* Add link back to PR in warning

* fix: add missing `return null` for `!allowUnregistered` code path

* Clean up message -- move more info to PR comment

* Add setTimeout to delay call debug warning call until after `app.forum` is defined

* Add backticks around data type

js/src/common/Store.ts

@@ -1,5 +1,6 @@
 import app from '../common/app';
 import { FlarumRequestOptions } from './Application';
+import fireDebugWarning from './helpers/fireDebugWarning';
 import Model, { ModelData, SavedModelData } from './Model';
 
 export interface MetaInformation {
@@ -120,11 +121,17 @@ export default class Store {
   pushObject<M extends Model>(data: SavedModelData, allowUnregistered: false): M;
   pushObject<M extends Model>(data: SavedModelData, allowUnregistered = true): M | null {
     if (!this.models[data.type]) {
-      if (allowUnregistered) {
-        return null;
+      if (!allowUnregistered) {
+        setTimeout(() =>
+          fireDebugWarning(
+            `[Flarum 2.0 Deprecation] Cannot push object of type \`${data.type}\`, as that type has not yet been registered in the store. This will throw an error in Flarum 2.0 and later.
+
+For more information, see https://github.com/flarum/core/pull/3206.`
+          )
+        );
       }
 
-      throw new Error(`Cannot push object of type ${data.type}, as that type has not yet been registered in the store.`);
+      return null;
     }
 
     const type = (this.data[data.type] = this.data[data.type] || {});