Fix conflict resolve strategies in project loading

CURA-4053

When loading a project file:

- Only check if the global stack exists to detect conflicts instead
  of checking the global stack and the extruder stacks. It can happen
  that the global stack exists while the extruder stacks not or the
  other way around.

- Always assign a resolve strategy to container(s). There can be
  "None" strategies and those were not handled correctly.

plugins/3MFReader/ThreeMFWorkspaceReader.py

@@ -129,6 +129,10 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
         instance_container_list = []
         material_container_list = []
 
+        resolve_strategy_keys = ["machine", "material", "quality_changes"]
+        self._resolve_strategies = {k: None for k in resolve_strategy_keys}
+        containers_found_dict = {k: False for k in resolve_strategy_keys}
+
         #
         # Read definition containers
         #
@@ -176,8 +180,10 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
                 container_id = self._stripFileToId(material_container_file)
                 materials = self._container_registry.findInstanceContainers(id=container_id)
                 material_labels.append(self._getMaterialLabelFromSerialized(archive.open(material_container_file).read().decode("utf-8")))
-                if materials and not materials[0].isReadOnly():  # Only non readonly materials can be in conflict
-                    material_conflict = True
+                if materials:
+                    containers_found_dict["material"] = True
+                    if not materials[0].isReadOnly():  # Only non readonly materials can be in conflict
+                        material_conflict = True
                 Job.yieldThread()
 
         # Check if any quality_changes instance container is in conflict.
@@ -205,6 +211,7 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
                 # Check if quality changes already exists.
                 quality_changes = self._container_registry.findInstanceContainers(id = container_id)
                 if quality_changes:
+                    containers_found_dict["quality_changes"] = True
                     # Check if there really is a conflict by comparing the values
                     if quality_changes[0] != instance_container:
                         quality_changes_conflict = True
@@ -227,21 +234,24 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
         # Load ContainerStack files and ExtruderStack files
         global_stack_file, extruder_stack_files = self._determineGlobalAndExtruderStackFiles(
             file_name, cura_file_names)
-        self._resolve_strategies = {"machine": None, "quality_changes": None, "material": None}
         machine_conflict = False
-        for container_stack_file in [global_stack_file] + extruder_stack_files:
-            container_id = self._stripFileToId(container_stack_file)
-            serialized = archive.open(container_stack_file).read().decode("utf-8")
-            if machine_name == "":
-                machine_name = self._getMachineNameFromSerializedStack(serialized)
-            stacks = self._container_registry.findContainerStacks(id = container_id)
-            if stacks:
-                # Check if there are any changes at all in any of the container stacks.
-                id_list = self._getContainerIdListFromSerialized(serialized)
-                for index, container_id in enumerate(id_list):
-                    if stacks[0].getContainer(index).getId() != container_id:
-                        machine_conflict = True
-            Job.yieldThread()
+        # Because there can be cases as follows:
+        #  - the global stack exists but some/all of the extruder stacks DON'T exist
+        #  - the global stack DOESN'T exist but some/all of the extruder stacks exist
+        # To simplify this, only check if the global stack exists or not
+        container_id = self._stripFileToId(global_stack_file)
+        serialized = archive.open(global_stack_file).read().decode("utf-8")
+        if machine_name == "":
+            machine_name = self._getMachineNameFromSerializedStack(serialized)
+        stacks = self._container_registry.findContainerStacks(id = container_id)
+        if stacks:
+            containers_found_dict["machine"] = True
+            # Check if there are any changes at all in any of the container stacks.
+            id_list = self._getContainerIdListFromSerialized(serialized)
+            for index, container_id in enumerate(id_list):
+                if stacks[0].getContainer(index).getId() != container_id:
+                    machine_conflict = True
+        Job.yieldThread()
 
         num_visible_settings = 0
         try:
@@ -301,10 +311,14 @@ class ThreeMFWorkspaceReader(WorkspaceReader):
         #  - new:       create a new container
         #  - override:  override the existing container
         #  - None:      There is no conflict, which means containers with the same IDs may or may not be there already.
-        #               If they are there, there is no conflict between the them.
-        #               In this case, you can either create a new one, or safely override the existing one.
+        #               If there is an existing container, there is no conflict between the them, and default to "override"
+        #               If there is no existing container, default to "new"
         #
         # Default values
+        for key, strategy in self._resolve_strategies.items():
+            if key not in containers_found_dict or strategy is not None:
+                continue
+            self._resolve_strategies[key] = "override" if containers_found_dict[key] else "new"
 
         return WorkspaceReader.PreReadResult.accepted