Fix type error

The typing was added by Nallath just now but it is giving an error for me.
The private access is allowed here because we're implementing it in the same class, even though it is a different instance.

This fixes the type error a bit more strictly than what lfei just did at the same time as me.

cura/LayerDataDecorator.py

@@ -1,9 +1,10 @@
-from typing import TYPE_CHECKING, Optional
+# Copyright (c) 2019 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
 
+from typing import Optional
 from UM.Scene.SceneNodeDecorator import SceneNodeDecorator
 
-if TYPE_CHECKING:
-    from cura.LayerData import LayerData
+from cura.LayerData import LayerData
 
 
 ## Simple decorator to indicate a scene node holds layer data.
@@ -15,10 +16,10 @@ class LayerDataDecorator(SceneNodeDecorator):
     def getLayerData(self) -> Optional["LayerData"]:
         return self._layer_data
 
-    def setLayerData(self, layer_data: Optional["LayerData"]) -> None:
+    def setLayerData(self, layer_data: LayerData) -> None:
         self._layer_data = layer_data
 
     def __deepcopy__(self, memo) -> "LayerDataDecorator":
         copied_decorator = LayerDataDecorator()
-        copied_decorator.setLayerData(self.getLayerData())
+        copied_decorator._layer_data = self._layer_data
         return copied_decorator