fixed selection operators to be compatible with zero-operator

nifty5/operators/selection_operator.py

@@ -20,6 +20,7 @@ from __future__ import absolute_import, division, print_function
 from ..compat import *
 from .linear_operator import LinearOperator
 from ..multi.multi_domain import MultiDomain
+from ..field import Field
 
 
 class SelectionOperator(LinearOperator):
@@ -52,7 +53,11 @@ class SelectionOperator(LinearOperator):
     def apply(self, x, mode):
         self._check_input(x, mode)
         if mode == self.TIMES:
-            return x[self._key]
+            if isinstance(x[self._key], Field):
+                return x[self._key]
+            else:
+                #else it is probably None
+                return Field.full(self.target, 0)
         else:
             from ..multi.multi_field import MultiField
             return MultiField.from_dict({self._key: x}, self._domain)