Fix 3D viewer omission of STEP models with relative path assignment

The project was not assigned to the board in the 3D viewer frame,
resulting in a failure to locate the relative path for STEP models.
To fix this, the current project should be assigned to the board in
the 3D viewer frame.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16242


(cherry picked from commit d2e51fdb5f)

c824484c Fix 3D viewer omission of STEP models with relative path assignment

Co-authored-by: Dhineshkumar S <[email protected]>

3d-viewer/3d_viewer/eda_3d_viewer_frame.cpp

@@ -570,6 +570,9 @@ void EDA_3D_VIEWER_FRAME::LoadSettings( APP_SETTINGS_BASE *aCfg )
     {
         applySettings( cfg );
 
+        if( !GetBoard()->GetProject() )
+            GetBoard()->SetProject( &Prj() );
+
         m_boardAdapter.SetBoard( GetBoard() );
 
         // When opening the 3D viewer, we use the OpenGL mode, never the ray tracing engine