fix dedicated server issue

Author: brachy84

src/main/java/com/cleanroommc/modularui/ClientProxy.java

@@ -31,7 +31,6 @@ void preInit(FMLPreInitializationEvent event) {
         super.preInit(event);
 
         MinecraftForge.EVENT_BUS.register(ClientEventHandler.class);
-        MinecraftForge.EVENT_BUS.register(GuiManager.class);
         MinecraftForge.EVENT_BUS.register(KeyBindHandler.class);
 
         if (ModularUIConfig.enabledTestGuis) {

src/main/java/com/cleanroommc/modularui/CommonProxy.java

@@ -32,6 +32,7 @@ public class CommonProxy {
 
     void preInit(FMLPreInitializationEvent event) {
         MinecraftForge.EVENT_BUS.register(CommonProxy.class);
+        MinecraftForge.EVENT_BUS.register(GuiManager.class);
 
         if (ModularUIConfig.enabledTestGuis) {
             MinecraftForge.EVENT_BUS.register(TestBlock.class);

src/main/java/com/cleanroommc/modularui/factory/GuiManager.java

@@ -111,6 +111,7 @@ public static void onTick(TickEvent.ServerTickEvent event) {
         }
     }
 
+    @SideOnly(Side.CLIENT)
     @SubscribeEvent
     public static void onGuiOpen(GuiOpenEvent event) {
         if (lastMui != null && event.getGui() == null) {