GPv3: Fix TreeNode::parent_group() for root group

For the root group, the parent is `nullptr` so calling `parent_group()`
would fail. This fixes the issue by adding a null check and returning
`nullptr` otherwise.

source/blender/blenkernel/intern/grease_pencil.cc

@@ -481,7 +481,10 @@ Layer &TreeNode::as_layer_for_write()
 
 LayerGroup *TreeNode::parent_group() const
 {
-  return &this->parent->wrap();
+  if (this->parent) {
+    return &this->parent->wrap();
+  }
+  return nullptr;
 }
 
 LayerMask::LayerMask()