I've been informed of an incompatibility between this and my mod TabbyChat. The simplest basic fix would be for you to not to change the chat gui and instead just changing the text field. The only thing you do in GuiSyntaxChat is initGui, so you can easily do that in your GuiOpenEvent. That would allow TabbyChat to work.
In order to get your text field to work with tabbychat, you will need to set the text field of my wrapper just once. Something like this should do it.
I'm teaching a Minecraft Command Blocks class, and your mod would be super-helpful. However, we're using 1.10! Would it be at all possible for me to get my hands on your source code so that I could update it (or for you to update it?) I'd gladly give you the updated code if necessary.