Only close if file is a file

Author: dschwoerer

xarray/backends/file_manager.py

@@ -238,7 +238,7 @@ def close(self, needs_lock: bool = True) -> None:
         with self._optional_lock(needs_lock):
             default = None
             file = self._cache.pop(self._key, default)
-            if needs_lock and self._lock:
+            if needs_lock and self._lock and file is not None:
                 with self._lock:
                     file.close()
             else: