Add missing not

pathml/core/slide_data.py

@@ -245,7 +245,7 @@ def tile_stride(self, value):
  self._tile_stride = value
 
  def get_tiles(self):
- if self.slide is None and not self._generated_tiles:
+ if self.slide is not None and not self._generated_tiles:
  self._generated_tiles = True
  for tile in self._generate_tiles():
  yield tile