Return undefined if cell or line doesn't exist

src/public/Terminal.ts

@@ -180,14 +180,19 @@ class BufferApiView implements IBufferApi {
   public get viewportY(): number { return this._buffer.ydisp; }
   public get baseY(): number { return this._buffer.ybase; }
   public get length(): number { return this._buffer.lines.length; }
-  public getLine(y: number): IBufferLineApi { return new BufferLineApiView(this._buffer.lines.get(y)); }
+  public getLine(y: number): IBufferLineApi | undefined { return new BufferLineApiView(this._buffer.lines.get(y)); }
 }
 
 class BufferLineApiView implements IBufferLineApi {
   constructor(private _line: IBufferLine) {}
 
   public get isWrapped(): boolean { return this._line.isWrapped; }
-  public getCell(x: number): IBufferCellApi { return new BufferCellApiView(this._line, x); }
+  public getCell(x: number): IBufferCellApi | undefined {
+    if (x < 0 && x >= this._line.length) {
+      return undefined;
+    }
+    return new BufferCellApiView(this._line, x);
+  }
   public translateToString(trimRight?: boolean, startColumn?: number, endColumn?: number): string {
     return this._line.translateToString(trimRight, startColumn, endColumn);
   }