config/module: tree.Child on a nil tree works

config/module/tree.go

@@ -66,6 +66,10 @@ func (t *Tree) Config() *config.Config {
 
 // Child returns the child with the given path (by name).
 func (t *Tree) Child(path []string) *Tree {
+	if t == nil {
+		return nil
+	}
+
 	if len(path) == 0 {
 		return t
 	}

config/module/tree_test.go

@@ -12,6 +12,11 @@ import (
 )
 
 func TestTreeChild(t *testing.T) {
+	var nilTree *Tree
+	if nilTree.Child(nil) != nil {
+		t.Fatal("child should be nil")
+	}
+
 	storage := testStorage(t)
 	tree := NewTree("", testConfig(t, "child"))
 	if err := tree.Load(storage, GetModeGet); err != nil {