diff --git a/cura/CuraApplication.py b/cura/CuraApplication.py
index 099c71c708..402917d748 100755
--- a/cura/CuraApplication.py
+++ b/cura/CuraApplication.py
@@ -1518,12 +1518,11 @@ class CuraApplication(QtApplication):
         """
         Checks if the given file URL is a valid project file.
         """
+        file_path = QUrl(file_url).toLocalFile()
+        workspace_reader = self.getWorkspaceFileHandler().getReaderForFile(file_path)
+        if workspace_reader is None:
+            return False  # non-project files won't get a reader
         try:
-            file_path = QUrl(file_url).toLocalFile()
-            workspace_reader = self.getWorkspaceFileHandler().getReaderForFile(file_path)
-            if workspace_reader is None:
-                return False  # non-project files won't get a reader
-
             result = workspace_reader.preRead(file_path, show_dialog=False)
             return result == WorkspaceReader.PreReadResult.accepted
         except Exception as e:
diff --git a/cura/Settings/ExtruderManager.py b/cura/Settings/ExtruderManager.py
index 351843ae14..b5f9a35914 100755
--- a/cura/Settings/ExtruderManager.py
+++ b/cura/Settings/ExtruderManager.py
@@ -49,6 +49,9 @@ class ExtruderManager(QObject):
     ##  Notify when the user switches the currently active extruder.
     activeExtruderChanged = pyqtSignal()
 
+    ## The signal notifies subscribers if extruders are added
+    extrudersAdded = pyqtSignal()
+
     ##  Gets the unique identifier of the currently active extruder stack.
     #
     #   The currently active extruder stack is the stack that is currently being
@@ -406,6 +409,7 @@ class ExtruderManager(QObject):
 
             if extruders_changed:
                 self.extrudersChanged.emit(global_stack_id)
+                self.extrudersAdded.emit()
                 self.setActiveExtruderIndex(0)
 
     ##  Get all extruder values for a certain setting.
diff --git a/plugins/3MFReader/ThreeMFWorkspaceReader.py b/plugins/3MFReader/ThreeMFWorkspaceReader.py
index 5c62c361c3..77a7da8b6a 100755
--- a/plugins/3MFReader/ThreeMFWorkspaceReader.py
+++ b/plugins/3MFReader/ThreeMFWorkspaceReader.py
@@ -168,11 +168,9 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
                 Logger.log("w", "Unknown definition container type %s for %s",
                            definition_container_type, each_definition_container_file)
             Job.yieldThread()
-        # sanity check
+
         if machine_definition_container_count != 1:
-            msg = "Expecting one machine definition container but got %s" % machine_definition_container_count
-            Logger.log("e", msg)
-            raise RuntimeError(msg)
+            return WorkspaceReader.PreReadResult.failed #Not a workspace file but ordinary 3MF.
 
         material_labels = []
         material_conflict = False
@@ -271,7 +269,6 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
         # if the global stack is found, we check if there are conflicts in the extruder stacks
         if containers_found_dict["machine"] and not machine_conflict:
             for extruder_stack_file in extruder_stack_files:
-                container_id = self._stripFileToId(extruder_stack_file)
                 serialized = archive.open(extruder_stack_file).read().decode("utf-8")
                 parser = configparser.ConfigParser()
                 parser.read_string(serialized)
@@ -303,7 +300,6 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
                         break
 
         num_visible_settings = 0
-        has_visible_settings_string = False
         try:
             temp_preferences = Preferences()
             serialized = archive.open("Cura/preferences.cfg").read().decode("utf-8")
diff --git a/plugins/CuraEngineBackend/CuraEngineBackend.py b/plugins/CuraEngineBackend/CuraEngineBackend.py
index 97463e07da..9713211ad3 100755
--- a/plugins/CuraEngineBackend/CuraEngineBackend.py
+++ b/plugins/CuraEngineBackend/CuraEngineBackend.py
@@ -88,6 +88,7 @@ class CuraEngineBackend(QObject, Backend):
         #
         self._global_container_stack = None
         Application.getInstance().globalContainerStackChanged.connect(self._onGlobalStackChanged)
+        Application.getInstance().getExtruderManager().extrudersAdded.connect(self._onGlobalStackChanged)
         self._onGlobalStackChanged()
 
         Application.getInstance().stacksValidationFinished.connect(self._onStackErrorCheckFinished)