Update app.py

app.py

@@ -78,8 +78,8 @@ def predict(seed,trunc_psi):
   return x,y,z,i,j,k
 
 def generate(seed,model_name,trunc_psi):
-
-    change_model(models[model_name])
+    if model_name:
+        change_model(models[model_name])
     x,y,z,i,j,k=predict(seed,trunc_psi)