Visitior amount can't be less than 0.

src/gui/simulator/VisitorsDialogPanel.java

@@ -89,10 +89,13 @@ public class VisitorsDialogPanel extends JDialog{
 	    
 	    private void save(){
 	            if(this.visitorstextfield.getText().equals("")){
-	                JOptionPane.showMessageDialog(null, "Visitors can't be empty!", "Warning", JOptionPane.WARNING_MESSAGE);
+					JOptionPane.showMessageDialog(null, "Visitors can't be empty!", "Warning", JOptionPane.WARNING_MESSAGE);
 	                return;
 	            }else{
 	            	int visitors = Integer.parseInt(this.visitorstextfield.getText());
+					if(visitors <0){
+						JOptionPane.showMessageDialog(null, "Visitors can't be less than 0!", "Warning", JOptionPane.WARNING_MESSAGE);
+					}
 	            	terrain.setMaxVisitors(visitors);
 	                dispose();
 	            }