closes #259 (Forbid deleting node with children)
closes #259 (Forbid deleting node with children)

mprinc/KnAllEdge,mprinc/KnAllEdge,Cha-OS/KnAllEdge,mprinc/KnAllEdge,Cha-OS/KnAllEdge,mprinc/KnAllEdge,Cha-OS/KnAllEdge,Cha-OS/KnAllEdge,Cha-OS/KnAllEdge,mprinc/KnAllEdge