Fix error about MessagePanel not being available for read markers
Fix error about MessagePanel not being available for read markers

Stacktrace:
```
TimelinePanel.js?b9ae:1139 Uncaught (in promise) TypeError: Cannot read property 'getBoundingClientRect' of null
    at Object._getLastDisplayedEventIndex (TimelinePanel.js?b9ae:1139)
    at Object.updateReadMarker (TimelinePanel.js?b9ae:751)
    at Object._callee$ (TimelinePanel.js?b9ae:613)
    at tryCatch (runtime.js?4422:45)
    at Generator.invoke [as _invoke] (runtime.js?4422:271)
    at Generator.prototype.<computed> [as next] (runtime.js?4422:97)
    at asyncGeneratorStep (asyncToGenerator.js?56ef:3)
    at _next (asyncToGenerator.js?56ef:25)
```
matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk