Steps to reproduce: (repro.mp4)
1. Make sure that the Editor has additional build support installed (e.g. Android/iOS)
2. Open any project
3. Open Build Settings
4. Select the installed platform support
5. Press the "Switch Platform" button multiple times
6. Notice that the 'Build' button activates, even though, the button label still states "Switch Platform" and the platform switching progress is not finished

Expected result: Build button should work only after the platform switching progress is completed and the button label is changed from "Switch Platform" to "Build"
Actual result: Build button is activated before the label is changed from "Switch Platform" to "Build" and before the platform switching process is finished

Reproducible with: 2018.3.0a9, 2018.4.0f1, 2019.2.0a14, 2019.3.0a2
Not reproducible with: 2017.4.26f1, 2018.3.0a8

Notes:
- the bug was introduced when the "Switch Platform" button was merged with the "Build" button
- reproducible on Windows, MacOS, Linux