Do not assume that canonical_transcript() returns a useful value.

Thanks Bronwen for the heads up.

modules/Bio/EnsEMBL/DBSQL/GeneAdaptor.pm

@@ -1101,8 +1101,12 @@ sub store {
 
   my $original             = $gene;
   my $original_transcripts = $gene->get_all_Transcripts();
-  my $old_canonical_transcript_id =
-    $gene->canonical_transcript()->dbID();
+  my $old_canonical_transcript_id;
+
+  if ( defined( $gene->canonical_transcript() ) ) {
+    $old_canonical_transcript_id =
+      $gene->canonical_transcript()->dbID();
+  }
 
   my $seq_region_id;
 
@@ -1206,6 +1210,7 @@ sub store {
     $transcript_adaptor->store( $new, $gene_dbID, $analysis_id );
 
     if (   !defined($new_canonical_transcript_id)
+         && defined($old_canonical_transcript_id)
          && defined( $old->dbID() )
          && $old->dbID() == $old_canonical_transcript_id )
     {