Timeline won't load more messages - `TypeError: Cannot read properties of null (reading 'getEvents')`

### Steps to reproduce

1. Switch to a room
1. Scroll up and start paginating
1. Get stuck with a loading indicator
- It was stuck the whole day. Every time I came back it wouldn't get further
1. Eventually pressed the up arrow to view messages and now it has a loading indicator on the bottom and top
1. Everything is broken now and I see errors in the console
- Screenshots in [rageshake](https://github.com/matrix-org/element-web-rageshakes/issues/14197)

```
TypeError: Cannot read properties of null (reading 'getEvents')
at TimelineWindow.getEvents (timeline-window.ts:378:37)
at TimelinePanel_TimelinePanel.getEvents (TimelinePanel.tsx:1412:59)
at TimelinePanel.tsx:534:73
```

### Outcome

#### What did you expect?

Timeline is able to scrollback and is able to recover.

#### What happened instead?

Timeline broken on both ends.

### Operating system

Windows 10

### Browser information

Chrome Version 103.0.5060.114

### URL for webapp

develop.element.io

### Application version

1519d5672e4c-react-0053f21286e8-js-32bb4b1fc464

### Homeserver

matrix.org

### Will you send logs?

Yes -> https://github.com/matrix-org/element-web-rageshakes/issues/14197

Read timeline-window.ts around TimelineWindow.getEvents and TimelinePanel.tsx around TimelinePanel.getEvents. First reproduce the stuck loading state while paginating in a room and inspect the reported null getEvents error. Done means the timeline can scroll back and recover without breaking at either end.

Written by the indexing model from the issue text.