Fix create epic in tree after selecting Add issue

ee/app/assets/javascripts/related_items_tree/store/mutations.js

 import Vue from 'vue';
 
 import * as types from './mutation_types';
+import { issuableTypesMap } from 'ee/related_issues/constants';
 
 export default {
   [types.SET_INITIAL_CONFIG](
@@ -142,6 +143,7 @@ export default {
   [types.TOGGLE_CREATE_EPIC_FORM](state, { toggleState }) {
     state.showCreateEpicForm = toggleState;
     state.showAddItemForm = false;
+    state.issuableType = issuableTypesMap.EPIC;
   },
 
   [types.TOGGLE_CREATE_ISSUE_FORM](state, { toggleState }) {

ee/changelogs/unreleased/195158-epic-tree-create-new-epic-creates-epic-that-it-thinks-is-an-iss.yml

+---
+title: Fix create epic in tree after selecting Add issue
+merge_request: 28300
+author:
+type: fixed

ee/spec/frontend/related_items_tree/store/mutations_spec.js

@@ -389,6 +389,7 @@ describe('RelatedItemsTree', () => {
 
           expect(state.showCreateEpicForm).toBe(data.toggleState);
           expect(state.showAddItemForm).toBe(false);
+          expect(state.issuableType).toEqual('epic');
         });
       });