Remove unused packages and fix invariant violation on AsyncStore onNotReady (#9404)

* Remove traces of browser-request & mocha

* Remove unused matrix-react-test-utils

* Fix dispatcher invariant violation

* Add null-guard

* Improve types

* Fix null-guard

* Fix issue with authed users going directly to /#/login

package.json

@@ -199,7 +199,6 @@
     "jest-raw-loader": "^1.0.1",
     "jest-sonar-reporter": "^2.0.0",
     "matrix-mock-request": "^2.5.0",
-    "matrix-react-test-utils": "^0.2.3",
     "matrix-web-i18n": "^1.3.0",
     "postcss-scss": "^4.0.4",
     "raw-loader": "^4.0.2",