Fixed 3mf writing

The type hinting changes also changed the way we handle certain imports, which caused saving to fail.

CURA-3215

plugins/3MFWriter/ThreeMFWriter.py

@@ -144,8 +144,7 @@ class ThreeMFWriter(MeshWriter):
                 translation_matrix.setByTranslation(translation_vector)
                 transformation_matrix.preMultiply(translation_matrix)
 
-
-            root_node = UM.Application.getInstance().getController().getScene().getRoot()
+            root_node = UM.Application.Application.getInstance().getController().getScene().getRoot()
             for node in nodes:
                 if node == root_node:
                     for root_child in node.getChildren():