This fix is needed in order for the new approach to create elements to work. Basically, the init function was called correctly when creating element through factory, but was not otherwise (for instance, when you simply pull an invokable form from service manager).
Maybe this could be moved in AbstractPluginManager for every class that implements InitializableInterface.
Fix hook in plugin manager
Remove unnecessary init in factory
Merge branch 'hotfix/3219' into develop