fix example launching before loading image

app.py

@@ -131,6 +131,9 @@ def generate(
 ) -> PIL.Image.Image:
     torch.manual_seed(seed)
 
+    if style_image_value is None:
+        gr.Error("Please provide a style image")
+
     if not use_negative_prompt:
         negative_prompt = None  # type: ignore
     if not use_prompt_2: