wxGUI/timeline: fix case when there are no registered datasets (merge from trunk, https://trac.osgeo.org/grass/changeset/60050)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@60051 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/timeline/frame.py

@@ -396,7 +396,8 @@ class TimelineFrame(wx.Frame):
                      for etype, maps in etypesDict.iteritems()]
                     for mapset, etypesDict in tDict.iteritems()]
         # flatten this list
-        allDatasets = reduce(lambda x, y: x + y, reduce(lambda x, y: x + y, allDatasets))
+        if allDatasets:
+            allDatasets = reduce(lambda x, y: x + y, reduce(lambda x, y: x + y, allDatasets))
 
         for dataset in datasets:
             errorMsg = _("Space time dataset <%s> not found.") % dataset