Media & Text - Block validation error occurs if media upload finishes when the post is closed

[pinarol]

Detected during testing #1503.
Also reproducible @ 9f4afd6 develop
Tested with iPhone 8 plus, iOS 13.1

• Add media & text block, tap placeholder and choose Choose from device
• Start upload but before it finishes close the post by selecting Update Post
• Wait until the upload finishes
• Re-open the post and you'll see:

• Switch to html mode, meda-text is still showing the upload id:

Expected:

• Add media & text block, tap placeholder and choose Choose from device
• Start upload but before it finishes close the post by selecting Update Post
• Wait until the upload finishes
• Re-open the post
• Verify that no red screen occurs
• Switch to html mode
• Verify that the mediaId is correct(usually a shorter and a positive number)

[koke]

Tested on WPAndroid develop with 1.15.1 and could reproduce the issue.

[etoledom]

Fixed via wordpress-mobile/WordPress-iOS#12797 and wordpress-mobile/WordPress-Android#10710