Use Plater's existing background_process() accessor

It was already public and used elsewhere, so the new get_background_process()
and its duplicate forward declaration were redundant; Plater.hpp is now untouched.

src/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -39,10 +39,9 @@ DesignCanvas::DesignCanvas(wxWindow* parent)
     m_canvas->allow_multisample(OpenGLManager::can_multisample());
     m_canvas->set_config(wxGetApp().plater()->config());
     m_canvas->set_model(&m_model);
-    // Reuse the editor's shared slicing process: GLCanvas3D::render() (via
-    // _max_bounding_box) dereferences the process when canvas type == View3D.
-    // Passing nullptr segfaults; this mirrors View3D/Preview/AssembleView.
-    m_canvas->set_process(wxGetApp().plater()->get_background_process());
+    // Nothing to slice here, but GLCanvas3D derefs the process unguarded every frame, so it
+    // cannot be null. Borrow the plater's, as the editor canvases do.
+    m_canvas->set_process(&wxGetApp().plater()->background_process());
     m_canvas->set_type(GLCanvas3D::ECanvasType::CanvasView3D);
 
     // CAD navigation, this canvas only: left-drag sweeps a selection rubber band, so orbit

src/slic3r/GUI/Plater.cpp

@@ -20649,11 +20649,6 @@ PartPlateList& Plater::get_partplate_list()
     return p->partplate_list;
 }
 
-BackgroundSlicingProcess* Plater::get_background_process()
-{
-    return &p->background_process;
-}
-
 void Plater::apply_background_progress()
 {
     PartPlate* part_plate = p->partplate_list.get_curr_plate();

src/slic3r/GUI/Plater.hpp

@@ -47,7 +47,6 @@ class Model;
 class ModelObject;
 class ModelInstance;
 class Print;
-class BackgroundSlicingProcess;
 class SLAPrint;
 //BBS: add partplatelist and SlicingStatusEvent
 class PartPlateList;
@@ -750,10 +749,6 @@ public:
 
     //BBS: partplate list related functions
     PartPlateList& get_partplate_list();
-    // Shared background slicing process (same instance View3D/Preview/AssembleView
-    // use). Exposed so the Design tab's native GLCanvas3D can be wired exactly like
-    // the editor canvases (GLCanvas3D::render() dereferences the process).
-    BackgroundSlicingProcess* get_background_process();
     void validate_current_plate(bool& model_fits, bool& validate_error);
     // Rebuild the missing-plugin sets from the active presets and (re)show/close their notifications.
     // Returns true when slicing must be blocked (a referenced plugin is still missing); sets