prevent IndexError if no GCP files

app/models/task.py

@@ -1074,10 +1074,10 @@ class Task(models.Model):
         :return: path to changed GCP file or None if no GCP file was found/changed
         """
         gcp_path = self.find_all_files_matching(r'.*\.txt$')
-        if len(gcp_path) == 0: return None
 
         # Skip geo.txt, image_groups.txt files
         gcp_path = list(filter(lambda p: os.path.basename(p).lower() not in ['geo.txt', 'image_groups.txt'], gcp_path))
+        if len(gcp_path) == 0: return None
 
         # Assume we only have a single GCP file per task
         gcp_path = gcp_path[0]