handles not existing RoomPowerLevels event gracefully

src/components/views/settings/AddPrivilegedUsers.tsx

@@ -52,6 +52,16 @@ export const AddPrivilegedUsers: React.FC<AddPrivilegedUsersProps> = ({ room, de
         const userIds = getUserIdsFromCompletions(selectedUsers);
         const powerLevelEvent = room.currentState.getStateEvents(EventType.RoomPowerLevels, "");
 
+        // `RoomPowerLevels` event should exist, but technically it is not guaranteed.
+        if (powerLevelEvent === null) {
+            Modal.createDialog(ErrorDialog, {
+                title: _t("Error"),
+                description: _t("Failed to change power level"),
+            });
+
+            return;
+        }
+
         try {
             await client.setPowerLevel(room.roomId, userIds, powerLevel, powerLevelEvent);
             setSelectedUsers([]);

test/components/views/settings/AddPrivilegedUsers-test.tsx

@@ -17,11 +17,12 @@ import React from 'react';
 import { act, fireEvent, render, waitFor } from '@testing-library/react';
 import userEvent from "@testing-library/user-event";
 import { mocked } from "jest-mock";
-import { RoomMember } from "matrix-js-sdk/src/matrix";
+import { RoomMember, EventType } from "matrix-js-sdk/src/matrix";
 
 import {
     getMockClientWithEventEmitter,
     makeRoomWithStateEvents,
+    mkEvent,
 } from "../../../test-utils";
 import MatrixClientContext from '../../../../src/contexts/MatrixClientContext';
 import {
@@ -56,6 +57,13 @@ describe('<AddPrivilegedUsers />', () => {
 
         return member;
     };
+    (room.currentState.getStateEvents as unknown) = (_eventType: string, _stateKey: string) => {
+        return mkEvent({
+            type: EventType.RoomPowerLevels,
+            content: {},
+            user: 'user_id',
+        });
+    };
 
     const getComponent = () =>
         <MatrixClientContext.Provider value={mockClient}>