Quick (annoying) thing. Whilst deploying a solution, you may see this error:
Error occurred in deployment step ‘Activate Features’: Key cannot be null. Parameter name: Key
This is unhelpfully connected to your Content Type definitions where you have FieldRefs and you’re closing them with the long tag, as opposed to the short tag, i.e.,
replace it with this to fix:
<FieldRef ID=”…” />
Thanks to Dhiraj.