While in the chat gui, if you press the key you've set for the keybind, it will attempt to render (and fail, of course). If you set your render key to a commonly used key, like ., this can become quite annoying since it states its error message in chat.

A simple fix would be an early return in BlockRenderer#onFrameStart:

if (FMLClientHandler.instance().isGUIOpen(GuiChat.class)) {
    return;
}