Display error when gitweb config is invalid

If the gitweb configuration in the gerrit.config file is invalid,
i.e. parameters are missing, Gerrit will set the gitweb config
object's type to null.  In this case the plugin throws NPE when
trying to access it.

Check for the type being null and display an error message.

Change-Id: Ibf4734337707e3c89640110fce09cb4572a4a778