Fix to Issue #40

Added check if index is in range before using it to index the list of
items.

src/controlP5/DropdownList.java

@@ -127,6 +127,7 @@ public class DropdownList extends Controller< DropdownList > implements ControlL
 				// n += itemRange; /* UP */
 				int index = ( int ) n + itemIndexOffset;
 
+				if (index < items.size()) {
 					Map m = items.get( index );
 	
 					switch ( _myType ) {
@@ -146,6 +147,7 @@ public class DropdownList extends Controller< DropdownList > implements ControlL
 						m.put( "state" , !ControlP5.b( m.get( "state" ) ) );
 						break;
 					}
+				}
 
 			}
 		}