Bugfix: zooming in empty layers preview (because of disabled background processing) crashed

lib/Slic3r/GUI/Plater/2DToolpaths.pm

@@ -164,6 +164,8 @@ sub new {
     EVT_MOUSEWHEEL($self, sub {
         my ($self, $e) = @_;
         
+        return if !$self->GetParent->enabled;
+        
         my $old_zoom = $self->_zoom;
         
         # Calculate the zoom delta and apply it to the current zoom factor
@@ -205,6 +207,8 @@ sub new {
 sub mouse_event {
     my ($self, $e) = @_;
     
+    return if !$self->GetParent->enabled;
+    
     my $pos = Slic3r::Pointf->new($e->GetPositionXY);
     if ($e->Entering && &Wx::wxMSW) {
         # wxMSW needs focus in order to catch mouse wheel events