Fixing issue raised in #4393 (#4438)

gym-unity/gym_unity/envs/__init__.py

         # Set action spaces
         if self.group_spec.is_action_discrete():
             branches = self.group_spec.discrete_action_branches
-            if self.group_spec.action_shape == 1:
+            if self.group_spec.action_size == 1:
                 self._action_space = spaces.Discrete(branches[0])
             else:
                 if flatten_branched:

gym-unity/gym_unity/tests/test_gym.py

     assert isinstance(env.action_space, spaces.MultiDiscrete)
 
 
+def test_action_space():
+    mock_env = mock.MagicMock()
+    mock_spec = create_mock_group_spec(
+        vector_action_space_type="discrete", vector_action_space_size=[5]
+    )
+    mock_decision_step, mock_terminal_step = create_mock_vector_steps(
+        mock_spec, num_agents=1
+    )
+    setup_mock_unityenvironment(
+        mock_env, mock_spec, mock_decision_step, mock_terminal_step
+    )
+
+    env = UnityToGymWrapper(mock_env, flatten_branched=True)
+    assert isinstance(env.action_space, spaces.Discrete)
+    assert env.action_space.n == 5
+
+    env = UnityToGymWrapper(mock_env, flatten_branched=False)
+    assert isinstance(env.action_space, spaces.Discrete)
+    assert env.action_space.n == 5
+
+
 @pytest.mark.parametrize("use_uint8", [True, False], ids=["float", "uint8"])
 def test_gym_wrapper_visual(use_uint8):
     mock_env = mock.MagicMock()