update app.py

app.py

@@ -6,21 +6,23 @@ def process_image(foreImg, flip, backImg, left, top, method, hscale, vscale, wid
 	if foreImg is None:
 		return None
 
-	bgwidth, bgheight = backImg.size
 	imwidth, imheight = foreImg.size
-	if method == 'horizontal scale factor':
-		scale = bgwidth * hscale / imwidth
-		imwidth = int(imwidth * scale)
-		imheight = int(imheight * scale)
-	elif method == 'vertical scale factor':
-		scale = bgheight * vscale / imheight
-		imwidth = int(imwidth * scale)
-		imheight = int(imheight * scale)
-	else:
-		if width > 0:
-			imwidth = width
-		if height > 0:
-			imheight = height
+	if not backImg is None:
+		if method == 'horizontal scale factor':
+			bgwidth, _ = backImg.size
+			scale = bgwidth * hscale / imwidth
+			imwidth = int(imwidth * scale)
+			imheight = int(imheight * scale)
+		elif method == 'vertical scale factor':
+			_, bgheight = backImg.size
+			scale = bgheight * vscale / imheight
+			imwidth = int(imwidth * scale)
+			imheight = int(imheight * scale)
+		else:
+			if width > 0:
+				imwidth = width
+			if height > 0:
+				imheight = height
 
 	if isLoc:
 		if backImg is None: