Fix NPE when open called twice
Fix NPE when open called twice

Fixes #2433

sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js