Fix None check order in _tree_layout (#3421)

* Fix None check order in _tree_layout

* add tests to test_graph.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Nikhil-42

manim/mobject/graph.py

@@ -112,13 +112,12 @@ def _tree_layout(
     vertex_spacing: tuple | None = None,
     orientation: str = "down",
 ):
-    children = {root_vertex: list(T.neighbors(root_vertex))}
-
-    if not nx.is_tree(T):
-        raise ValueError("The tree layout must be used with trees")
     if root_vertex is None:
         raise ValueError("The tree layout requires the root_vertex parameter")
+    if not nx.is_tree(T):
+        raise ValueError("The tree layout must be used with trees")
 
+    children = {root_vertex: list(T.neighbors(root_vertex))}
     # The following code is SageMath's tree layout implementation, taken from
     # https://github.com/sagemath/sage/blob/cc60cfebc4576fed8b01f0fc487271bdee3cefed/src/sage/graphs/graph_plot.py#L1447
 

tests/module/mobject/test_graph.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import pytest
+
 from manim import DiGraph, Graph, Scene, Text, tempconfig
 
 
@@ -100,3 +102,15 @@ def test_custom_animation_mobject_list():
         scene.play(G.animate.remove_vertices(2))
         assert str(G) == "Undirected graph on 3 vertices and 0 edges"
         assert scene.mobjects == [G]
+
+
+def test_tree_layout_no_root_error():
+    with pytest.raises(ValueError) as excinfo:
+        G = Graph([1, 2, 3], [(1, 2), (2, 3)], layout="tree")
+    assert str(excinfo.value) == "The tree layout requires the root_vertex parameter"
+
+
+def test_tree_layout_not_tree_error():
+    with pytest.raises(ValueError) as excinfo:
+        G = Graph([1, 2, 3], [(1, 2), (2, 3), (3, 1)], layout="tree", root_vertex=1)
+    assert str(excinfo.value) == "The tree layout must be used with trees"