fix: guard load_object_volume against objects with no instances (crash opening some MakerWorld 3MFs)

[BenJule]

Problem

Opening certain MakerWorld 3MFs crashes BambuStudio while loading. From the crash report in #11016 (macOS, Apple Silicon):

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000

0   ???                 0x0  ???
1   GLVolumeCollection::load_object_volume(...)
2   Plater::update_obj_preview_origin_thumbnail(...)
3   ObjColorPanel::generate_origin_thumbnail()
4   ObjColorPanel::ObjColorPanel(...)
...
9   GUI_App::handle_web_request(...)   // opened from MakerWorld

Root cause

GLVolumeCollection::load_object_volume dereferences model_object->instances[instance_idx] and later calls instance->get_transformation() without validating the index:

const ModelInstance *instance = model_object->instances[instance_idx];
...
v.set_instance_transformation(instance->get_transformation());   // crash if instance is garbage

Plater::update_obj_preview_origin_thumbnail (and update_obj_preview_thumbnail) call this with a hard-coded instance_idx = 0 for every object. An object that has no instance yet during loading makes instances[0] undefined behaviour → a null/garbage ModelInstance* → the jump-to-0x0 crash above.

Fix

A defensive guard at the top of load_object_volume: return early (-1, "no volume added") when model_object is null or volume_idx / instance_idx are out of range. The three call sites either pass valid indices or ignore the return value, so the healthy path is unchanged.

Fixes #11016

It may also be the cause of the intermittent MakerWorld-3MF import crash in #11025, though I have not confirmed that from a stack trace.

[lanewei120]

thanks

[guanyun-gudujian]

approved