Cast each container in their stack to actual types

We know for sure that these containers have those types. We'll accept the risk here that this assumption was wrong.

Contributes to issue CURA-5330.

cura/Settings/CuraContainerStack.py

@@ -1,7 +1,7 @@
 # Copyright (c) 2018 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
-from typing import Any, List, Optional, Union
+from typing import Any, cast, List, Optional, Union
 from PyQt5.QtCore import pyqtProperty, pyqtSignal, QObject
 
 from UM.Application import Application
@@ -73,7 +73,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The user changes container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setUserChanges, notify = pyqtContainersChanged)
     def userChanges(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.UserChanges]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.UserChanges])
 
     ##  Set the quality changes container.
     #
@@ -86,7 +86,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The quality changes container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setQualityChanges, notify = pyqtContainersChanged)
     def qualityChanges(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.QualityChanges]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.QualityChanges])
 
     ##  Set the quality container.
     #
@@ -99,7 +99,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The quality container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setQuality, notify = pyqtContainersChanged)
     def quality(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.Quality]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.Quality])
 
     ##  Set the material container.
     #
@@ -112,7 +112,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The material container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setMaterial, notify = pyqtContainersChanged)
     def material(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.Material]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.Material])
 
     ##  Set the variant container.
     #
@@ -125,7 +125,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The variant container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setVariant, notify = pyqtContainersChanged)
     def variant(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.Variant]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.Variant])
 
     ##  Set the definition changes container.
     #
@@ -138,7 +138,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The definition changes container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(InstanceContainer, fset = setDefinitionChanges, notify = pyqtContainersChanged)
     def definitionChanges(self) -> InstanceContainer:
-        return self._containers[_ContainerIndexes.DefinitionChanges]
+        return cast(InstanceContainer, self._containers[_ContainerIndexes.DefinitionChanges])
 
     ##  Set the definition container.
     #
@@ -151,7 +151,7 @@ class CuraContainerStack(ContainerStack):
     #   \return The definition container. Should always be a valid container, but can be equal to the empty InstanceContainer.
     @pyqtProperty(QObject, fset = setDefinition, notify = pyqtContainersChanged)
     def definition(self) -> DefinitionContainer:
-        return self._containers[_ContainerIndexes.Definition]
+        return cast(DefinitionContainer, self._containers[_ContainerIndexes.Definition])
 
     @override(ContainerStack)
     def getBottom(self) -> "DefinitionContainer":