The function `_gen_shape_list` is using the resource `Res<Mesh>`, but during the import phase the resource used is instead `Ref<EditorSceneImporterMesh>`.
Note: the `Ref<EditorSceneImporterMesh>` is an intermediate resource, that will be used to create a `Res<Mesh>` at the end of the import process. `Ref<EditorSceneImporterMesh>` and `Ref<Mesh>` are not inheriting each other, so the internal cast done by `Ref<>` during the assignment, is always null:
```c++
Ref<EditorSceneImporterMesh> import_mesh(/* Assume it's initialized */);
CRASH_NOW(import_mesh.is_null());
Ref<Mesh> mesh = import_mesh;
CRASH_NOW(mesh.is_null()); // <--- Here we have a crash, since it's impossible perform the above cast, and the `mesh` is always null.
```
Here the full list of call to `_gen_shape_list`, where we can notice that a `Ref<EditorSceneImporterMesh>` is passed:
- https://github.com/AndreaCatania/godot/blob/master/editor/import/resource_importer_scene.cpp#L428
- https://github.com/AndreaCatania/godot/blob/master/editor/import/resource_importer_scene.cpp#L454-L458
- https://github.com/AndreaCatania/godot/blob/master/editor/import/resource_importer_scene.cpp#L512-L516
As you can notice, we always pass the following mesh: `Ref<EditorSceneImporterMesh> mesh = mi->get_mesh();`.
We already have the function `_pre_gen_shape_list` that executes the exact same job but using the correct type; Since there is no further usage of the function `_gen_shape_list` in the code base, I think it's just some leftover code, so I removed it entirely to use the proper function.
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