Fatal error: Call to a member function GetContent() on a non-object in

I have two main product category items - schoolware and footwear. I have followed the checkbox options tutorials for the schoolware products size options being passed to the shopping cart. Implemented successfully.

While implementing this to the footwear products I received the above error. I followed some forum threads suggesting rearranging the sequence of the code.

The above fatal error is now no longer being displayed and during some testing I received the following error:

Warning: session_start() [function.session-start]: Cannot send session
cache limiter - headers already sent (output started at
/Applications/XAMPP/xamppfiles/htdocs/h_schoolwear_specialists_rwd/
footwearDetails.php:9) in
/Applications/XAMPP/xamppfiles/htdocs/h_schoolwear_specialists_rwd/
WA_eCart/WA_eCart_Definition_PHP.php on line 465

When testing the footwear size options being added to the cart, the footwear page gets reloaded and only displays the size options that were selected originally, but does not redirect and add these to the cart page.

However, when I test the schoolwear size options, the schoolwear size option items are being added to the shopping cart as well as the previously selected footwear size options are being listed on the shopping cart page that were selected while testing

Both the schoolwear and footwear product items is sharing the same shopping cart.

I am attaching three pages that I use while testing, namely: schoolwearDetails, footwearDetails and cart-quote.

I have run out of options and thank you in advance for the assistance with this issue.