Why does whiptail bash script not working in %post or %pre section of RHEL7 Kickstart ?

Issue

• whiptail used with bash in %post section doesn't work with Red Hat Enterprise Linux 7 Kickstart installation.

%post
set -x
exec 2>> /root/post.log
{
echo "Removing unwanted rpm's"
yum remove -y a
echo 5
yum remove -y b
echo 10
yum remove -y c
echo 15
yum remove -y d
echo 20
yum remove -y e
echo 25
yum remove -y f
echo 30
yum remove -y g
echo 35
yum remove -y h
echo 40
yum remove -y i
echo 45
yum remove -y j
echo 100
} | whiptail --gauge "Running post installation scripts"  6 60 0 

• During post installation, still whiptail progress bar is not visible, only default "Running post installation script " message is displayed.
• whiptail Inputbox gets stuck during kickstart, when trying to type letters are white on black background
  -Yes/no box does not react