Fails to start after Chrome downgrade

We had a crash report ([internal link](https://github.com/matrix-org/element-web-rageshakes/issues/25971)) on Chrome with errors in the logs like this:

```
2024-05-21T06:58:48.335Z E Error starting matrixclient store - falling back to memory store Cannot read properties of null (reading 'nextBatch')
TypeError: Cannot read properties of null (reading 'nextBatch')
```

and this:

```
2024-05-21T06:58:50.813Z E Unable to load session invalid type: null, expected a sequence at line 1 column 4
Error: invalid type: null, expected a sequence at line 1 column 4
```

## Symptoms

I think the symptoms of this would be that the page would remain blank when opening Element Web, and refreshing the page would not help.

## Cause

After reading the code, we believe that these error messages could only be caused if multiple pieces of data in IndexedDB were replaced by `null` values. The fact that at least two separate object stores were affected lead us to believe this is a browser behaviour rather than a bug in Element Web. We found no plausible code paths that would insert null values into these parts of the DB.

We found https://issues.chromium.org/issues/41308609 , which does suggest that nulls might be inserted **if the user opened the application in an earlier version of Chrome after having previously opened it into a later version**. This problem would persist even if the user switched back to the later version.

I am creating this issue for visibility, not necessarily expecting a fix within Element Web since we suspect it is caused by a Chrome bug.

## Workaround

A workaround for this issue is to clear the storage for Element Web entirely. (See e.g. [How to Clear Storage and Site Data for a Single Site on Google Chrome](https://www.howtogeek.com/664912/how-to-clear-storage-and-site-data-for-a-single-site-on-google-chrome/)). This will delete your cryptographic identity on this device and if you have no other devices you will lose access to your old messages.