Undefined Array Key warnings

[Plofkat]

Added error handling to several lines (line numbers changed a bit as I was tinkering)
These seemed to only occur on tables that was not yet set up with displayAs()

Line 2090
Undefined array key warning

replace:
if ($this->category_required[$field] != TRUE){

with:

if (isset($this->category_required[$field]) && $this->category_required[$field] !== TRUE){

Line 1554
Undefined Array key

replace:

if ($this->displayAs_array[$field] != ''){

with:

if (isset($this->displayAs_array[$field]) && $this->displayAs_array[$field] != '') {

Line 1972
Undefined Array key

replace:

if ($this->displayAs_array[$field] != ''){

with:

if (isset($this->displayAs_array[$field]) && $this->displayAs_array[$field] != '') {