Fix memory mapping on Windows (#193)

* Fix memory mapping on Windows

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

sheeprl/utils/memmap.py

@@ -14,8 +14,6 @@
 import numpy as np
 from numpy.typing import DTypeLike
 
-from sheeprl.utils.imports import _IS_WINDOWS
-
 
 def is_shared(array: np.ndarray) -> bool:
     return isinstance(array, np.ndarray) and hasattr(array, "_mmap")
@@ -214,12 +212,12 @@ def from_array(
 
     def __del__(self) -> None:
         """Delete the memory-mapped array. If the memory-mapped array has ownership of the file and no other
-        reference to the memory-mapped array exists or the OS is Windows-based,
+        reference to the memory-mapped array exists,
         then the memory-mapped array will be flushed to disk and both the memory-mapped array and
         the file will be closed. If the memory-mapped array is mapped to a temporary file then the file is
         removed.
         """
-        if (self._has_ownership and getrefcount(self._file) <= 2) or _IS_WINDOWS:
+        if self._array is not None and self._has_ownership and getrefcount(self._file) <= 2:
             self._array.flush()
             self._array._mmap.close()
             del self._array._mmap