Fixed object sinking into print bed after parts import

lib/Slic3r/GUI/Plater/ObjectPartsPanel.pm

@@ -378,6 +378,7 @@ sub on_btn_load {
         }
     }
 
+    $self->{model_object}->center_around_origin if $self->{parts_changed};
     $self->_parts_changed;
 }
 
@@ -480,6 +481,7 @@ sub on_btn_delete {
         $self->{parts_changed} = 1;
     }
 
+    $self->{model_object}->center_around_origin if $self->{parts_changed};
     $self->_parts_changed;
 }