Because that's exactly what happens, once you have removed the original radio app. It first asked me which app I wanted to open this action, so I selected NavRadio+ and then instead of NavRadio+ opening.
View attachment 5853601
The Car Launcher reset to the default one, which is the one above: same happens when trying to use the FM widget. My unit don't have any physical buttons, so I need to use the ones on the touchscreen.
Default launcher obviously doesn't contain any FM shortcut, or widgets: but the one I prefer to use, does have them. And although I hardly use the FM radio. I think it's nice if the built-in shortcuts where working.
View attachment 5853603
Just for reference, because I don't have a screenshot of the car launcher I'm using now: but the Radio button, will not work anymore after removing the default radio. Although the music widget on these Launchers are also pretty useless, when you use Spotify.
View attachment 5853609
This launcher I use, and on the left side there is bar with in the middle a FM Radio icon. Once you remove the built-in FM radio, then these buttons are crippled and using them will cause the Launcher to crash and reset to the default one.
And if you don't remove the built-in FM app, then it will conflict with NavRadio+ so that's no option.