ArgumentError (Invalid argument: Instance of 'TextEditingController')
Solution 1:
This line:
Firestore.instance.collection('reg').add({'name':controllerName})
should be replaced with:
Firestore.instance.collection('reg').add({'name':controllerName.text})
Also, you should probably give your TextField an initial value of an empty string so that it can't be null.
Solution 2:
controllerName is not a String,
controllerName.text
use that