Fix NPE in reflow
Fix NPE in reflow

See microsoft/vscode#72149

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