String viewName = "Problems";
    // This will work fine if the view is already opened in your workbench layout.
    // Otherwise, you will have to "simulate" clicking on the "Window -> Show View -> Problem" workbench menu so that the view is indeed displayed in the workbench.
    SWTBotView view = getWorkbench().getBot().viewByTitle(viewName);
    view.show();
    view.bot().tree();
    String category = "Errors";
    int nbOfErrors = 0;
    for (SWTBotTreeItem item : tree.getAllItems()) {
      String text = item.getText();
      if (text != null && text.startsWith(category)) {
        item.expand();
        nbOfErrors = item.getNodes().size();
        break;
      }
   }