Add MatrixClientPeg::safeGet and use it in tests (#10985)

test/stores/right-panel/RightPanelStore-test.ts

@@ -36,7 +36,7 @@ describe("RightPanelStore", () => {
     let cli: MockedObject<MatrixClient>;
     beforeEach(() => {
         stubClient();
-        cli = mocked(MatrixClientPeg.get());
+        cli = mocked(MatrixClientPeg.safeGet());
         DMRoomMap.makeShared(cli);
 
         // Make sure we start with a clean store