The issue is in the non-existent function call being made there: "checkOkButtonState();" instead of "checkButtonState();". The latter validates the user entered values (property name, hour and minute) and based on the outcome enables or disables dialogue's "OK" button. Basically, we've got a typo here.
Steps to reproduce:
1. Log in as any User;
2. Head to their Home space;
3. Click "More actions" icon.
4. Click "Manage Content Rules" link.
5. Click "Create Rule" link.
6. Select "Items with specific date value in property" option and click "Set Values and add" button.
7. Set any value into "Property name" input field > "OK" button becomes enabled.
8. Enter a non-digit value into or remove current value from hour (or minute) field.
"OK" button becomes disabled.
"OK" button is still enabled. Once clicked in this case will lead to a System Error (java.lang.NullPointerException).