Fix for build error and perhaps the grayed out arrange button

src/libslic3r/SLA/SLASupportTree.cpp

@@ -781,6 +781,7 @@ public:
         model_height = bb.max(Z);
 
         meshcache_valid = true;
+        return meshcache;
     }
 
     double full_height() const {

src/slic3r/GUI/Plater.cpp

@@ -996,6 +996,7 @@ Plater::priv::priv(Plater *q, MainFrame *main_frame) :
     sidebar(new Sidebar(q)),
     canvas3D(GLCanvas3DManager::create_wxglcanvas(notebook))
 {
+    arranging.store(false);
     background_process.set_fff_print(&print);
 	background_process.set_sla_print(&sla_print);
     background_process.set_gcode_preview_data(&gcode_preview_data);