Fix various issues with the loading screen and datagen

Fix NPE on datagen startup
Fix early loading GUI hanging and never terminating after datagen
Add loading GUI messages for datagen

patches/minecraft/net/minecraft/data/DataGenerator.java.patch

@@ -9,7 +9,15 @@
     }
  
     public Collection<Path> func_200389_a() {
-@@ -53,6 +53,10 @@
+@@ -38,6 +38,7 @@
+ 
+       for(IDataProvider idataprovider : this.field_200396_d) {
+          field_200393_a.info("Starting provider: {}", (Object)idataprovider.func_200397_b());
++         net.minecraftforge.fml.StartupMessageManager.addModMessage("Generating: " + idataprovider.func_200397_b());
+          stopwatch1.start();
+          idataprovider.func_200398_a(directorycache);
+          stopwatch1.stop();
+@@ -53,6 +54,10 @@
        this.field_200396_d.add(p_200390_1_);
     }
  

src/fmllauncher/java/net/minecraftforge/fml/loading/progress/ClientVisualization.java

@@ -274,6 +274,7 @@ class ClientVisualization implements EarlyProgressVisualization.Visualization {
     @Override
     public Runnable start() {
         initWindow();
+        renderThread.setDaemon(true); // Don't hang the game if it terminates before handoff (i.e. datagen)
         renderThread.start();
         return org.lwjgl.glfw.GLFW::glfwPollEvents;
     }

src/main/java/net/minecraftforge/fml/ModLoader.java

@@ -292,7 +292,7 @@ public class ModLoader
         dataGeneratorConfig = new GatherDataEvent.DataGeneratorConfig(mods, path, inputs, serverGenerators, clientGenerators, devToolGenerators, reportsGenerator, structureValidator);
         existingFileHelper = new ExistingFileHelper(existingPacks, structureValidator);
         gatherAndInitializeMods(() -> {});
-        dispatchAndHandleError(LifecycleEventProvider.GATHERDATA, Runnable::run, null);
+        dispatchAndHandleError(LifecycleEventProvider.GATHERDATA, Runnable::run, () -> {});
         dataGeneratorConfig.runAll();
     }