NPE in TextRenderLayer.onGridChanged

[Tyriar]

VS Code saw this, the xterm.js version was somewhere in between 3.4 and 3.5.

Stack 
TypeError: Cannot read property 'beginFrame' of undefined
at TextRenderLayer.onGridChanged (\node_modules.asar\vscode-xterm\lib\renderer\TextRenderLayer.js:143:24)
/vs/workbench/parts/terminal/electron-browser/terminalInstance.ts#481:25
at \node_modules.asar\vscode-xterm\lib\renderer\Renderer.js:154:60
at Array.forEach (native)
at Renderer._renderRows (\node_modules.asar\vscode-xterm\lib\renderer\Renderer.js:154:28)
at RenderDebouncer._innerRefresh (\node_modules.asar\vscode-xterm\lib\utils\RenderDebouncer.js:29:14)
at \node_modules.asar\vscode-xterm\lib\utils\RenderDebouncer.js:24:88

VS Code issue: microsoft/vscode#51663