Attempt to set option only for existing stream

This resolves the bug when single audio stream cannot be exported to
the file due to fact that the output has no video stream.

Author: SuslikV

src/windows/export.py

@@ -835,12 +835,15 @@ def accept(self):
             # These extra options should be set in an extra method
             # No feedback is given to the user
             # TODO: Tell user if option is not available
-            # Set the quality in case crf was selected
-            if "crf" in self.txtVideoBitRate.text():
-                w.SetOption(openshot.VIDEO_STREAM, "crf", str(int(video_settings.get("video_bitrate"))) )
-
-            # Muxing options for mp4/mov
-            w.SetOption(openshot.VIDEO_STREAM, "muxing_preset", "mp4_faststart")
+            if export_type in [_("Audio Only")]:
+                # Muxing options for mp4/mov
+                w.SetOption(openshot.AUDIO_STREAM, "muxing_preset", "mp4_faststart")
+            else:
+                # Muxing options for mp4/mov
+                w.SetOption(openshot.VIDEO_STREAM, "muxing_preset", "mp4_faststart")
+                # Set the quality in case crf was selected
+                if "crf" in self.txtVideoBitRate.text():
+                    w.SetOption(openshot.VIDEO_STREAM, "crf", str(int(video_settings.get("video_bitrate"))) )
 
             # Open the writer
             w.Open()