regression: old Rich Replies bug causes react-gemini-scrollbar to somehow lose its reference to its scrollbar and NPEing

[ara4n]

I'm going to guess this is somehow related to #6068, but for the last few times i've unslept my laptop, I've had the app completely frozen and crashes like:

Switching to room id !fILrmqIZQfJcrUiBRk:matrix.org at event undefined
10:07:18.868 bundle.js:12677 Uncaught TypeError: Cannot read property 'update' of null
    at Object.componentDidUpdate (bundle.js:12677)
    at measureLifeCyclePerf (bundle.js:309331)
    at bundle.js:309984
    at CallbackQueue.notifyAll (bundle.js:118670)
    at ReactReconcileTransaction.close (bundle.js:314359)

in the log. In practice, bundle.js:12677 is:

	    componentDidUpdate: function componentDidUpdate() {
	        this.scrollbar.update();
	    },

...within react-gemini-scrollbar.

Will rageshake.

[turt2live]

Fwiw, I've only seen this when a rich quote crashes the app.

[lukebarnard1]

Closing because this only affects the old rich replies feature.