wrap starmapper call in a list() to ensure it finishes

Author: JoshKarpel

pymoo/model/problem.py

@@ -368,7 +368,7 @@ def func(_x):
             params = [[X[k], calc_gradient, self._evaluate, args, kwargs] for k in range(len(X))]
 
             starmapper = _params[0]
-            ret = starmapper(evaluate_in_parallel, params)
+            ret = list(starmapper(evaluate_in_parallel, params))
 
         elif _type == "threads":