[models] small update

Co-authored-by: Stephan Rave <stephanrave@uni-muenster.de>

src/pymor/models/basic.py

@@ -122,9 +122,10 @@ def _compute_output_d_mu(self, solution, mu, return_array=False, use_adjoint=Non
         else:
             assert self.output_functional is not None
             assert self.output_functional.linear
+            output_functional_array = self.output_functional.H.as_range_array(mu)
             dual_solutions = self.operator.range.empty()
             for d in range(self.output_functional.range.dim):
-                dual_problem = self.with_(operator=self.operator.H, rhs=self.output_functional.H.as_range_array(mu)[d])
+                dual_problem = self.with_(operator=self.operator.H, rhs=output_functional_array[d])
                 dual_solutions.append(dual_problem.solve(mu))
             gradients = [] if return_array else {}
             for (parameter, size) in self.parameters.items():