It's no longer possible to remove a material that is in use

CURA-2168

cura/Settings/ContainerManager.py

@@ -225,6 +225,15 @@ class ContainerManager(QObject):
 
         return result
 
+    @pyqtSlot(str, result = bool)
+    def isContainerUsed(self, container_id):
+        UM.Logger.log("d", "Checking if container %s is currently used in the active stacks", container_id)
+        for stack in cura.Settings.ExtruderManager.getInstance().getActiveGlobalAndExtruderStacks():
+            if container_id in [child.getId() for child in stack.getContainers()]:
+                UM.Logger.log("d", "The container is in use by %s", stack.getId())
+                return True
+        return False
+
     ##  Get a list of string that can be used as name filters for a Qt File Dialog
     #
     #   This will go through the list of available container types and generate a list of strings

resources/qml/Preferences/MaterialsPage.qml

@@ -104,7 +104,7 @@ UM.ManagementPage
         {
             text: catalog.i18nc("@action:button", "Remove");
             iconName: "list-remove";
-            enabled: base.currentItem != null && !base.currentItem.readOnly
+            enabled: base.currentItem != null && !base.currentItem.readOnly && !Cura.ContainerManager.isContainerUsed(base.currentItem.id)
             onClicked: confirmDialog.open()
         },
         Button