fixed getter issue of the second momentum

lbmpy/methods/conservedquantitycomputation.py

@@ -227,9 +227,12 @@ class DensityVelocityComputation(AbstractConservedQuantityComputation):
             main_assignments.extend([Assignment(lhs, sum(d[i] * pdf for d, pdf in zip(self._stencil, pdfs)))
                                      for i, lhs in enumerate(moment1_output_symbol)])
         if 'moment2' in output_quantity_names_to_symbols:
-            for p_i in self._symbolsOrder2:
-                main_assignments.append(Assignment(p_i, eqs[p_i]))
-                del eqs[p_i]
+            moment2_output_symbol = output_quantity_names_to_symbols['moment2']
+            if isinstance(moment2_output_symbol, Field):
+                moment2_output_symbol = moment2_output_symbol.center_vector
+            for i, p in enumerate(moment2_output_symbol):
+                main_assignments.append(Assignment(p, eqs[self._symbolsOrder2[i]]))
+                del eqs[self._symbolsOrder2[i]]
 
         ac = ac.copy(main_assignments, [Assignment(a, b) for a, b in eqs.items()])
         return ac.new_without_unused_subexpressions()