Check that MultiDiscrete.dtype is not None (#1196)

gymnasium/spaces/multi_discrete.py

@@ -59,6 +59,19 @@ def __init__(
             seed: Optionally, you can use this argument to seed the RNG that is used to sample from the space.
             start: Optionally, the starting value the element of each class will take (defaults to 0).
         """
+        # determine dtype
+        if dtype is None:
+            raise ValueError(
+                "MultiDiscrete dtype must be explicitly provided, cannot be None."
+            )
+        self.dtype = np.dtype(dtype)
+
+        #  * check that dtype is an accepted dtype
+        if not (np.issubdtype(self.dtype, np.integer)):
+            raise ValueError(
+                f"Invalid MultiDiscrete dtype ({self.dtype}), must be an integer dtype"
+            )
+
         self.nvec = np.array(nvec, dtype=dtype, copy=True)
         if start is not None:
             self.start = np.array(start, dtype=dtype, copy=True)
@@ -70,7 +83,7 @@ def __init__(
         ), "start and nvec (counts) should have the same shape"
         assert (self.nvec > 0).all(), "nvec (counts) have to be positive"
 
-        super().__init__(self.nvec.shape, dtype, seed)
+        super().__init__(self.nvec.shape, self.dtype, seed)
 
     @property
     def shape(self) -> tuple[int, ...]: