Fix: Dispose stream and clear document on RichTextPreview unload

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-4S2

Steps used to test these changes

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1. Verified that the RichTextPreview control now disposes of the stream associated with it when unloaded.
2. Confirmed that the document content in the RichTextPreview is cleared when the control is unloaded.
3. Tested the defensive check to ensure that loading is prevented if the stream is null or disposed.
4. Verified fix generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 2416057. Click here to continue debugging with Seer.

[hishitetsu]

I plan to verify this carefully when I have time, but at first glance, I don't think this fix will resolve the issue.

[hishitetsu]

I'm not sure if this will resolve the issue. Rather than that, I think it's better to simply catch the EndOfStreamException to avoid crashes.

[yaira2]

Where can we catch the EndOfStreamException?

[hishitetsu]

Where can we catch the EndOfStreamException?

In TextPreviewControl_Loaded.

[yaira2]

@hishitetsu thank you for your help. I pushed this change to main.