Tabs do not return to their initial state after closing a crashed split tab

[Softvision-CristinaBadescu]

[Notes]:

• I've encountered this issue naturally (one tab crashed), but it's also reproducible if you force the crash.
• The future opened pages' UI is affected: the middle bar is displayed.

[Affected versions]:

• Nightly 60.0a1
• Tab Split 0.1.9

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a clean Firefox profile with the latest Tab Split v0.1.9 installed.
• Have one tab open.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Click the "Tab Split" button.
3. Crash one of the tabs and switch to the other one.
4. Return to the crashed tab and close it.
   bonus
5. Click "Split tab" button and observe the behavior.

[Expected result]:
Step 4 - The tab returns to its initial state.
Step 5 - The tab is split.

[Actual result]:
Step 4 - The other tab remains split but the 2nd part of the page is blank.
Step 5 - Nothing happens.

[Additional notes]:

• This issue is also reproducible for a split that consists of websites, only if other tabs aren't open.
• Once this happens the tab's colored outline also remains.
• Here is a screen recording with this issue: