Terminator crash when change selection of layout in layout editor

[Sean Wang]

Problem:
            1. When open preference layout editor, If you select a leaf item of a layout on the right panel
            2. Then change to select another layout name on the left, the terminator will crash.
    Reason:
            1. When changing the layout name in the left panel, the TreeStore of the LayoutEditor will be cleared by calling treestore.clear() in LayoutEditor.setLayout().
            2.When the LayoutEditor.treestore.clear(), LayoutEditor.on_layout_item_selection_changed() while be triggered.
            3. Since the layout name has been changed, it can't find an old layout_item in a new layout in LayoutEditor.set_layout_item().
Environment:
    Distributor ID: Ubuntu
    Description: Ubuntu 21.04
    Release: 21.04
    Codename: hirsute
Version: 2.1.0