Multiple fixes for Site menu item modal

[jsorohova]

Included fixes:

• Correct parent displaying for all item types (it was not working for Media items)
• Removed the item itself from the list of possible parents
• Fixed error handling on item type change

[github-actions]

Accessibility Insights Action: All applicable checks passed

• URLs: 13 URL(s) passed, and 0 were not scannable
• Rules: 29 check(s) passed, and 23 were not applicable
• Download the Accessibility Insights artifact to view the detailed results of these checks

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This scan used axe-core 4.3.2 with Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36.