replaygain: Fix #1518, GStreamer missing plugins

beetsplug/replaygain.py

@@ -388,6 +388,12 @@ class GStreamerBackend(Backend):
         self._res = self.Gst.ElementFactory.make("audioresample", "res")
         self._rg = self.Gst.ElementFactory.make("rganalysis", "rg")
 
+        if self._src is None or self._decbin is None or self._conv is None \
+           or self._res is None or self._rg is None:
+            raise FatalReplayGainError(
+                "Failed to load required GStreamer plugins"
+            )
+
         # We check which files need gain ourselves, so all files given
         # to rganalsys should have their gain computed, even if it
         # already exists.

docs/changelog.rst

@@ -66,6 +66,8 @@ Fixes:
 * Some messages in the console UI now use plural nouns correctly. Thanks to
   :user:`JesseWeinstein`. :bug:`1521`
 * Sorting numerical fields (such as track) now works again. :bug:`1511`
+* :doc:`/plugins/replaygain`: Missing GStreamer plugins now cause a helpful
+  error message instead of a crash. :bug:`1518`
 
 
 1.3.13 (April 24, 2015)