**BUG**
Concise problem statement:
Steps to reproduce bug:
1. pressed and held down cntl-z to undo everything
2. I was trying to move a portion of my circut back
3.
Results: The program crashed
Expected results: I wanted to move a portion of my schematic to where it is more readable
Url:<you url here>(click the Share my works button to get it)
Browser:
errMsg:#browser: chrome;
#type: undefined;
#string: NotFoundError: Failed to execute 'replaceChild' on 'Node': The new child element is null.;
#stack: Error: Failed to execute 'replaceChild' on 'Node': The new child element is null.
at Error (native)
at Object.Bc.$o0102 (https://easyeda.com/lib/min/editorSCH.min.js:5:20280)
at Object.Bc.$o0101 (https://easyeda.com/lib/min/editorSCH.min.js:6:2922)
at Object.Bc.$o0100 (https://easyeda.com/lib/min/editorSCH.min.js:4:19550)
at Object.Bc.go (https://easyeda.com/lib/min/editorSCH.min.js:6:2268)
at jc.registerHook.undo (https://easyeda.com/lib/min/editorSCH.min.js:3:23716)
at jc (https://easyeda.com/lib/min/editorSCH.min.js:1:80)
at HTMLDocument.n (https://easyeda.com/lib/min/editorSCH.min.js:3:333)
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
