Merge pull request #1381 from Tyriar/1379_viewport_throw

Fix exception when reset is called before open

src/Terminal.ts

@@ -2169,7 +2169,9 @@ export class Terminal extends EventEmitter implements ITerminal, IInputHandlingT
     this._customKeyEventHandler = customKeyEventHandler;
     this._inputHandler = inputHandler;
     this.refresh(0, this.rows - 1);
-    this.viewport.syncScrollArea();
+    if (this.viewport) {
+      this.viewport.syncScrollArea();
+    }
   }