[pitivi] previewers: Never try to dot a `None` pipeline

[Thibault Saunier <tsaunier src gnome org>]

commit 664946508da2f95012c865ef7a41a54c87343de3
Author: Thibault Saunier <tsaunier gnome org>
Date:   Sat Nov 25 16:27:26 2017 -0300

    previewers: Never try to dot a `None` pipeline

 pitivi/timeline/previewers.py |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/pitivi/timeline/previewers.py b/pitivi/timeline/previewers.py
index 26ab468..cdf7bf6 100644
--- a/pitivi/timeline/previewers.py
+++ b/pitivi/timeline/previewers.py
@@ -1004,9 +1004,10 @@ class AudioPreviewer(Previewer, Zoomable, Loggable):
                 self._launchPipeline()
                 self.becomeControlled()
             else:
-                Gst.debug_bin_to_dot_file_with_ts(self.pipeline,
-                                                  Gst.DebugGraphDetails.ALL,
-                                                  "error-generating-waveforms")
+                if self.pipeline:
+                    Gst.debug_bin_to_dot_file_with_ts(self.pipeline,
+                                                      Gst.DebugGraphDetails.ALL,
+                                                      "error-generating-waveforms")
                 self.error("Aborting due to waveforms generation issue: %s",
                            message.parse_error())