Projects using GWT 1.5 or lower may only specify one entry point module to compile

Despite

Having the 2.0.3 GWT plugin installed

Never having used the 1.5 GWT plugin

I was getting the following error when clicking the GWT compile button :

 Projects using GWT 1.5 or lower may only specify one entry point module to compile

The solution was to move my GWT dependencies above my Maven dependencies.

Right click on the project -> Build Path -> Configure Build Path... -> Order and Export

Click on GWT SDK and Up until it is just below the JDK.