[bug] Class is not a valid Dashboard controller

[dmitryuk]

Closes #6787 #6792 #6646

\EasyCorp\Bundle\EasyAdminBundle\Cache\CacheWarmer initializes DashboardControllerRegistry before cache warmed, so on first cold run DashboardControllerRegistry always threw an exception "XXX" class is not a valid Dashboard controller.

User story:

• remove var/cache folder
• Run example Console command with URL generation AdminUrlGenerator::generateUrl

[javiereguiluz]

Thanks Alexander!

while merging I did some minor tweaks (#51947f4870ce80c49339b9ce0f002934ca781f2f). Your code was perfectly correct, but in Symfony it's more common to use nullable properties that you check and then call the method to initialize them instead of using a flag property like $init. But both approaches are equally fine. Thanks!