PlanningEntityDescriptor does not support write-only properties

The following code:

            Method propertyGetter = propertyDescriptor.getReadMethod();
            if (propertyGetter.isAnnotationPresent(PlanningVariable.class)) {

in method processPropertyAnnotations() assumes that properties always have getters. However, the method getReadMethod() might indeed return null if the property is write-only, as explained in the JavaBeans(TM) Specification 1.01 Final Release. A null-check should therefore be added in order to prevent NullPointerExceptions.