Improve mod loading error message for errors inside mod constructors (#9751)

javafmllanguage/src/main/java/net/minecraftforge/fml/javafmlmod/FMLModContainer.java

@@ -21,6 +21,8 @@ import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.Marker;
 import org.apache.logging.log4j.MarkerManager;
 
+import java.lang.reflect.InvocationTargetException;
+import java.util.Objects;
 import java.util.Optional;
 
 public class FMLModContainer extends ModContainer
@@ -70,6 +72,11 @@ public class FMLModContainer extends ModContainer
         }
         catch (Throwable e)
         {
+            // When a mod constructor throws an exception, it's wrapped in an InvocationTargetException which hides the
+            // actual exception from the mod loading error screen.
+            if (e instanceof InvocationTargetException wrapped)
+                e = Objects.requireNonNullElse(wrapped.getCause(), e); // unwrap the exception
+
             LOGGER.error(LOADING,"Failed to create mod instance. ModID: {}, class {}", getModId(), modClass.getName(), e);
             throw new ModLoadingException(modInfo, ModLoadingStage.CONSTRUCT, "fml.modloading.failedtoloadmod", e, modClass);
         }