Fix unit tests

Signed-off-by: Adam Li <adam2392@gmail.com>

treeple/neighbors.py

@@ -7,6 +7,7 @@
 from sklearn.neighbors import NearestNeighbors
 from sklearn.utils.validation import check_is_fitted, validate_data
 
+from treeple.tree import DecisionTreeClassifier
 from treeple.tree._neighbors import _compute_distance_matrix, compute_forest_similarity_matrix
 
 
@@ -31,13 +32,19 @@ class NearestNeighborsMetaEstimator(BaseEstimator, MetaEstimatorMixin):
         The number of parallel jobs to run for neighbors, by default None.
     """
 
-    def __init__(self, estimator, n_neighbors=5, radius=1.0, algorithm="auto", n_jobs=None):
+    def __init__(self, estimator=None, n_neighbors=5, radius=1.0, algorithm="auto", n_jobs=None):
         self.estimator = estimator
         self.n_neighbors = n_neighbors
         self.algorithm = algorithm
         self.radius = radius
         self.n_jobs = n_jobs
 
+    def get_estimator(self):
+        if self.estimator is not None:
+            return DecisionTreeClassifier(random_state=0)
+        else:
+            return copy(self.estimator)
+
     def fit(self, X, y=None):
         """Fit the nearest neighbors estimator from the training dataset.
 
@@ -58,7 +65,7 @@ def fit(self, X, y=None):
         """
         X, y = validate_data(self, X, y, accept_sparse="csc")
 
-        self.estimator_ = copy(self.estimator)
+        self.estimator_ = self.get_estimator()
         try:
             check_is_fitted(self.estimator_)
         except NotFittedError: