LibGUI: Make GTreeView inherit from GAbstractColumnView

This makes GTreeView able to support multi-column models!

Only one column can be the "tree column", this is column 0 by default
but can be changed by overriding GModel::tree_column().

1 files changed, 1 insertions, 1 deletions

diff --git a/Libraries/LibGUI/GModel.cpp b/Libraries/LibGUI/GModel.cpp
index d1c4e94d35..96834b4d4c 100644
--- a/Libraries/LibGUI/GModel.cpp
+++ b/Libraries/LibGUI/GModel.cpp
@@ -42,7 +42,7 @@ GModelIndex GModel::create_index(int row, int column, const void* data) const
 GModelIndex GModel::sibling(int row, int column, const GModelIndex& parent) const
 {
     if (!parent.is_valid())
-        return {};
+        return index(row, column, {});
     int row_count = this->row_count(parent);
     if (row < 0 || row > row_count)
         return {};