removed negative values, and fixed error

app.py

@@ -22,7 +22,7 @@ class BinPacking:
         self.width = width
         self.height = height
         self.pgrp = None
-        self.total = None
+        self.total = 0
         self.image = cv2.resize(image, (int(image.shape[1]*imageScale), int(image.shape[0]*imageScale)))
         if self.image.shape[2] == 4:
             x,y = np.where(self.image[:,:,3]==0)
@@ -57,7 +57,10 @@ class BinPacking:
 
         max_item = int(self.width*self.height/item.area)
         self.pgrp = nest([item,]*max_item, self.box)
-        self.total = len(self.pgrp[0])
+        if self.pgrp is None or len(self.pgrp) == 0:
+            self.total = 0
+        else:
+            self.total = len(self.pgrp[0])
         return self
 
     def visualize(self):
@@ -75,7 +78,7 @@ class BinPacking:
             ),
             paper_bgcolor="LightSteelBlue",
         )
-        if self.pgrp == None:
+        if self.pgrp == None or len(self.pgrp) == 0:
             return fig
 
         _, thresh = cv2.threshold(self.imgray, 127, 255, 0)
@@ -137,7 +140,7 @@ if __name__ == "__main__":
                 
             with gr.Column():
                 plot_output = gr.Plot()
-                text = gr.Text()
+                text = gr.Text(label="Message")
                 
         fit.click(fn=process, inputs=[width, height, scale, image], outputs=[plot_output, text])