Fix division error in inactive leaf size estimation (#1721)

* Fix division error in inactive leaf size estimation

* Change line

* Add check for zero devision

* Add release notes

* unreleased.md aktualisieren

Co-authored-by: Émile <73942755+e10e3@users.noreply.github.com>

---------

Co-authored-by: Émile <73942755+e10e3@users.noreply.github.com>

Author: kulbachcedric

docs/releases/unreleased.md

@@ -4,3 +4,7 @@
 
 - Added `update_many` method to `stats.PearsonCorr`.
 - Changed the calculation of the Kuiper statistic in `base.KolmogorovSmirnov` to correspond to the reference implementation. The Kuiper statistic uses the difference between the maximum value and the minimum value.
+
+## tree
+
+- Added handling for division by zero in `tree.hoeffding_tree` for leaf size estimation.

river/tree/hoeffding_tree.py

@@ -296,9 +296,9 @@ def _estimate_model_size(self):
                 total_active_size += calculate_object_size(leaf)
             else:
                 total_inactive_size += calculate_object_size(leaf)
-        if total_active_size > 0:
+        if total_active_size > 0 and self._n_active_leaves > 0:
             self._active_leaf_size_estimate = total_active_size / self._n_active_leaves
-        if total_inactive_size > 0:
+        if total_inactive_size > 0 and self._n_inactive_leaves > 0:
             self._inactive_leaf_size_estimate = total_inactive_size / self._n_inactive_leaves
         actual_model_size = calculate_object_size(self)
         estimated_model_size = (