DB version: 11.2.0.3 on Linux
APEX: 4.2
APEX Listener 1.1.4 on tomcat6
Firefox 16.0.2
Radio Group NULL values behave differently than on APEX 4.1.1.
In 4.2, when creating Radio Group and selecting:
Display Null Value=Yes
Null Display Value=No value (or any string)
Null Return Value= (leave empty, to get NULL returned)
When user selects that "No value" option, then APEX item value is set to "on", but on APEX 4.1, the item value was set to NULL as expected.
When looking at the webpage source code, the reason seems to be, that APEX does not generate value attribute for the NULL value (so browser returns default value "on"):
<input type="radio" id="P1_X_0" name="p_t01" ><label for="P1_X_0">No value</label>