Guard against returning null icon.

Somehow, the icon can be null, at least in dev.  This follows the practice of using `Objects.requireNonNull` like in `DummyParentMod`.  It will still crash but maybe something will be clearer.

Author: gniftygnome

src/main/java/com/terraformersmc/modmenu/util/mod/fabric/FabricMod.java

@@ -184,12 +184,12 @@ public FabricMod(ModContainer modContainer, Set<String> modpackMods) {
 				defaultIconWarning = false;
 			}
 
-			return iconHandler.createIcon(
+			return Objects.requireNonNull(iconHandler.createIcon(
 				FabricLoader.getInstance()
 					.getModContainer(ModMenu.MOD_ID)
 					.orElseThrow(() -> new RuntimeException("Cannot get ModContainer for Fabric mod with id " + ModMenu.MOD_ID)),
 				"assets/" + ModMenu.MOD_ID + "/unknown_icon.png"
-			);
+			));
 		}
 		return icon;
 	}