Fix for dynamic select lists generated from contents of input file when no valid datasets exist in the history.

lib/galaxy/tools/parameters/dynamic_options.py

@@ -363,6 +363,7 @@ class DynamicOptions( object ):
         if self.dataset_ref_name:
             dataset = other_values.get( self.dataset_ref_name, None )
             assert dataset is not None, "Required dataset '%s' missing from input" % self.dataset_ref_name
+            if not dataset: return [] #no valid dataset in history
             options = self.parse_file_fields( open( dataset.file_name ) )
         else:
             options = list( self.file_fields )