Parameters entered in Set Parameters tab are not propagated to anonymous block.
During execution of PL/SQL package from the Schema browser by clicking the right mouse button on the package, and clicking "Execute Package", the window "Set Parameters" appeared. Then values was entered at the tab "Arguments" but not all entered parameters appeared in the tab "Anonymous Block Code". The tab "Anonymous Block Code" shows "NULL" values for these variables.
It is necessary to check check box "Expression" next to the check box "Null". See the screenshot with example below.