Forms :: FRM-40010 / Cannot Read Form

created a form which is calling 3 different forms based on the radio button. The main form say 'MAIN.fmb' has the code " execute_query " in when-radio-changed trigger and the following:

if :BLOCK.RADIOGRPNAME = '1'
then open_form('XX');
else if :BLOCK.RADIOGRPNAME = '2'
then open_form('YY');
else if :BLOCK.RADIOGRPNAME = '3'
then open_form('ZZ');
end if;
end if;
end if;

in the pre-query trigger of the control block where XX,YY,ZZ are the three corresponding forms.

While clicking the radio button, 'm getting the error FRM-40010:cannot read form XX.fmb.

the .fmb, .fmx both exist in the location and individually all the forms are working fine but while being called from MAIN is showing this error.