UI Issue with block if UIHint.Textarea decorated string property text is too long

Description

Description of the issue:
If you have a block with a string property decorated with [UIHint(UIHint.Textarea)], and you paste a very long text into the area before you create/save the block, the top bar with the Create button disappears.

Steps to reproduce
1. Using Alloy, on the contentarea of a page, click "create a new block" and choose a "Jumbotron block".
2. Paste a long text (using lorem ipsum) into the "Image description" property.
3. Click outside the image description property and then back in the image description property. (Might have to click back and forth a couple of times.)

Expected:
Nothing special should happen.

Actual:
The top bar with the Create button and Name field disappears.