"Tab Split" button does not work at first click after installation in previously opened windows

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Tab Split 0.1.7

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have 2 browser windows open.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Install the "Tab Split" add-on through "about:debugging" page.
3. Switch to the other window.
4. Click the "Tab Split" button and observe the behavior.

[Expected result]:

• The tab is split.

[Actual result]:

• Nothing happens.

[Notes]:

• In the Browser Console, a "win.TabSplit is undefined" message is displayed. Here is a screenshot:
  

• Here is a screen recording with this issue: