Hello NeilMS11,
Thank you for using Apple Support Communities! I understand that when sharing a video from the Photos app to the Mail app, the app is crashing on your iPhone X. I would be happy to help provide steps to take next so you can share your videos again.
Let's start by following the steps outlined in the support article below, making sure your iPhone has been updated to iOS 13.2:
If an app unexpectedly quits, stops responding, or won’t open - Apple Support
I would also like to have you test if you're able to insert the video directly from the Mail app:
How to send attachments in Mail on your iPhone, iPad, and iPod touch - Apple Support
Could you also further clarify if it is the Photos app or the Mail app that's quitting back to the Home screen?
Cheers!