Fix issue of shouting error when removing a node. - It shout a Python traceback message when removing a node that is referenced by a svg:use node. - when the node been removed, the node would be duplicated. The duplication was used to replace the svg:use one. - The ID of inserted duplication is the same as duplicated node, and modified to a new and unique ID. - Original design of domview_monitor can not handle it. - It is resolved by changing domview_monitor._id2node from single mapping to multiple mapping one.

<inkscape-extension>
    <name>Convert to Symbol</name>
    <id>MadButterfly.ConvertSymbol</id>
	<dependency type="executable" location="extensions">AssignSymbol.py</dependency>
	<dependency type="executable" location="extensions">inkex.py</dependency>
    <effect>
		<object-type>group</object-type>
		<effects-menu>
			<submenu _name="MadButterfly"/>
		</effects-menu>
    </effect>
    <script>
        <command reldir="extensions" interpreter="python">AssignSymbol.py</command>
    </script>
</inkscape-extension>