Object reference not set to an instance of an object
This error happens when you try to use a property or call a method of an object that is null. More details: here
A simple use of Visual studio DEBUGGER can tell you the object because of which it is happening. Just look at the stack trace and put a debugger on that line. Check the objects of that line and see if any one is null and you are trying to use that objects property. Handle the same.
For now, do look at this to start with assuming value for 'Gender' is getting through:
Enabled='<%# Eval("Gender") %>'
Eval will get the string equivalent and not the boolean one. Setting it to a Boolean expecting property might cause issues.
BTW, I could not understand the point of having a enable proerty being set via an Eval here. That too when Panel is made hidden!