Null check xterm viewport

Fixes microsoft#135867

Author: Tyriar

src/vs/workbench/contrib/terminal/browser/terminalInstance.ts

@@ -1176,7 +1176,7 @@ export class TerminalInstance extends Disposable implements ITerminalInstance {
 			// change since the number of visible rows decreases.
 			// This can likely be removed after https://github.com/xtermjs/xterm.js/issues/291 is
 			// fixed upstream.
-			this._xtermCore.viewport._innerRefresh();
+			this._xtermCore.viewport?._innerRefresh();
 		}
 	}
 

src/vs/workbench/contrib/terminal/browser/xterm-private.d.ts

@@ -10,7 +10,7 @@ import { IBufferCell } from 'xterm';
 export type XTermAttributes = Omit<IBufferCell, 'getWidth' | 'getChars' | 'getCode'> & { clone?(): XTermAttributes };
 
 export interface XTermCore {
-	viewport: {
+	viewport?: {
 		_innerRefresh(): void;
 	};
 	_onKey: IEventEmitter<{ key: string }>;