This is an intentional addition, as mentioned above:

> @glouppe 204b0aa fixes an issue I encountered when grid searching the AdaBoost and tree parameters over a dataset with some negative weights (real data, part of my research). Seems like in some obscure conditions every possible split ends up producing children with one having an overall negative weight (after boosting where some positive weights are suppressed). The ValueError "Attempting to find a split with a negative ..." was raised (at a depth of 3) thus terminating the grid search. I think in this situation it is safe to just make this node a leaf and terminate splitting this branch (done in 204b0aa).