pushHandleLayers method Null safety
Marks this handler to be responsible for pushing LeaderLayers for the selection handles.
This handler is responsible for pushing the leader layers with the given layer links if they are not null. It is possible that only one layer is non-null if this handler is only responsible for pushing one layer link.
startHandle needs to be placed at the visual location of selection
endHandle needs to be placed at the visual location of selection
end. Typically, the visual locations should be the same as
void pushHandleLayers(LayerLink? startHandle, LayerLink? endHandle);