The issue appears to be that the transform manager adds a series of IUI objects apart from the request to add the text component itself. It adds a total of 3 objects; the original request, another one (which I gather you use to probably solve some scaling issues), and then a third; it is this third object that causes the debugger to balk with an index out of range error. Interestingly, when not run in debug, the app proceeds, no doubt without adding the third object; this needs to be handled evidently.
Preliminarily, this code fixes it. You just check to make sure that the index is available before you add the object, if it isn't, just return null. Although I haven't gone all the way throught it, the stack must check for that null value and short circuit the operation, so the error is handled and the app can continue to run cleanly.
I don't know if this will effect the app otherwise, but at this point it seems to be working.
At the FlexTransformManager class, at line 255, edit the function like so:
override public function addChildAt($child:DisplayObject, $index:int):DisplayObject
{
var mc : DisplayObject;
if ( $index < super.getChildren ( ).length )
{
mc = super.addChildAt($child, $index);
autoAddChild($child);
}
return mc;
}