Easier to reproduce using Unity Hub or QA multi-tool
Steps to reproduce:
1. Open any project
2. Open Build Settings window
3. Select a platform that is not installed (e.g. Android)
4. Don't close the editor and install that component
5. After installation, notice that "Switch Platform" button is enabled
Expected result: Switch Platform button is not enabled and/or a warning to restart the Editor is thrown
Actual result: Switch Platform button is enabled and pressing it changes the current target icon (repro.mp4)
Reproducible with: 2017.4.0f1, 2017.4.12f1, 2018.2.13f1, 2018.3.0b7, 2019.1.0a5