Fix bug in source selection

sksurgerydavinci/ui/sksurgerydavinci_demo.py

@@ -35,10 +35,11 @@ def run_demo(args):
 
     if num_of_input_sources == 1:
 
-        if len(video_sources[0]) > 1:
+        if isinstance(video_sources[0], str):
             # it is a filename
             source = video_sources[0]
             print(f"Loading video from {source}")
+
         else:
             source = int(video_sources[0])