fix convertor

Author: VaghinakDev

src/superannotate/lib/core/usecases/annotations.py

@@ -592,7 +592,10 @@ def __init__(
 
     def validate_project_type(self):
         if self._project.project_type != constances.ProjectType.VIDEO.value:
-            raise AppException("The function only supports video projects.")
+            raise AppException(
+                "The function is not supported for"
+                f" {constances.ProjectType.get_name(self._project.project_type)} projects."
+            )
 
     def execute(self):
         self.reporter.disable_info()

src/superannotate/lib/core/video_convertor.py

@@ -112,10 +112,11 @@ def get_median(self, annotations: List[dict]) -> dict:
                 median_annotation = annotation
         return median_annotation
 
-    def merge_first_frame(self, frames_mapping):
+    @staticmethod
+    def merge_first_frame(frames_mapping):
         try:
             if 0 in frames_mapping:
-                frames_mapping[1].extend(frames_mapping[0])
+                frames_mapping[1].extendleft(frames_mapping[0])
                 del frames_mapping[0]
         finally:
             return frames_mapping