[Android] The app crashes if multiple "Sign In" windows are opened and you try to go back to the home screen

[Softvision-MariusComan]

[Affected Platforms]:

• Android 8.0.0 (Google Pixel)
• Android 8.0.0 (Samsung S8)
• Android 6.0 (Huawei P8 Lite)

[Prerequisites]:

• Have the latest version of the "Firefox Notes" application (app-release-signed) custom built on 8/1/2018 installed.
• You are not logged into "Firefox Notes".

[Steps to reproduce]:

1. Open the Notes application and tap the "Sign In" button.
2. Tap the system's "Back" button and quickly tap the "Sign In" button again.
3. Tap the system's "Back" button after the e-mail field is displayed.
4. Tap the "Back" button again and observe the behavior.

[Expected result]:

• You are returned to the "Login" screen.

[Actual result]:

• The app crashes.

[Notes]:

• Attached a screen recording of the issue:
  

[vladikoff]

Should be fixed via 689a4fe

[Softvision-RemusDranca]

I have verified this issue using the latest "Firefox Notes" application (app-release-signed) custom built on 8/21/2018 (https://circleci.com/gh/mozilla/notes/2938) and the issue is still reproducible.
Tested on Android 8.0.0 (Google Pixel); Android 8.0.0 (Samsung S8) and Android 6.0 (Huawei P8 Lite).
Considering this I will reopen the issue.