code: private void validateTabbed(int tabIndex) {
  		if (!isTabbed()) {
  			throw new IllegalArgumentException("Not initialised as a tabbed dialog - must use method without tab parameters");
  		}
  		if (tabIndex < 0 || tabIndex >= this.tabPanels.size()) {
  			throw new IllegalArgumentException("Invalid tab index: " + tabIndex);
  		}
  	}
docs: Validates that the dialogue is tabbed and the given tab index is valid.
  
  @param tabIndex the index of the tab to validate.
  @throws IllegalArgumentException if the dialogue was not initialised with tabs or if no tab exists with the given index.
  @see #validateNotTabbed()