The problem has been solved by adding exceptions for case which POS hasn't handled any transactions. 
================================================================================public void checkForHighestSalesProductAndPercentage() {
		try {
			if (salesFunction.checkSalesIsEmpty()) {
				throw new ExNoSalesExists();
			} else {
				int daysUnit = getCheckSalesDaysUnit();
				int ageGroupUnit = getCheckAgeGroupDigit();
				boolean ageFilter = (ageGroupUnit == 0) ? false : true;
				Product product = salesFunction.printHighestSalesProduct(daysUnit, ageFilter, ageGroupUnit);
				if (product == null) {
					throw new ExNoSalesInSelectedAge();
				} else {
					double percentage = salesFunction.getSalesPercentageForProduct(daysUnit, product, ageFilter,
							ageGroupUnit, salesFunction.getTotalSalesNum());
					System.out.printf("The highest sales product for %s is %s\n", tools.returnStrforMoments(daysUnit),
							product.getName());
					System.out.printf("The Sales Percentage for %s in %s is %.2f %%\n", product.getName(),
							tools.returnStrforMoments(daysUnit), percentage);
				}
			}
		} catch (ExNoSalesExists e) {
			System.out.println(e.getMessage());
		} catch (ExNoSalesInSelectedAge e) {
			System.out.println(e.getMessage());
		}

	}
================================================================================
Please input your Worker ID for logging in the system...
1
Please input your Member ID for logging in the system...
1234
---Welcome to Morx Supermarket---
---To continue, please proceed your actions---
Input (1) for accessing Inventory System
Input (2) for accessing Checkout System
Input (3) for accessing Sales System
Input (4) for accessing Membership System
Input (5) for switching member
Input (6) to exit
3
---Hi ON9, Welcome to Sales System.
------Input commands for further management---
Input (1) for Listing Sales
Input (2) for checking Total income
Input (3) for checking highest sales Product
Input (4) for exit
3
*** No Sales Exists! ***