[Kyttias]

It's quite bizarre that two other people have this mod installed on their site, other than myself, just fine and never experienced fatal errors. Yet, no matter how much I search, I can't find any helpful reference to a similar error that they would have fixed elsewhere. My classes/class_guicontainer.php and classes/class_guicomponent.php files identical to the default install!

EDIT: I can actually recreate your error... if you followed the instructions wrong, you replaced your entire view/inventoryview.php file instead of replacing only the index() function as instructed. Try replacing your entire file with this (or go back and follow the instructions more carefully):

  Spoiler: replace inventoryview.php 

PHP Code:

<?php

use Resource\Collection\LinkedList;

class InventoryView extends View{
    
    public function index(){
        $mysidia = Registry::get("mysidia");
        $document = $this->document;
        $document->setTitle($mysidia->lang->inventory);
        
        $inventory = $this->getField("inventory");
        
        $document->add(new Comment(" <style>
                .sc_item {
                  display: inline-table;
                  padding: 5px;
                  text-align: center;
                  font-family: 'Trebuchet MS', Helvetica, sans-serif;
                  font-size: 14px;
                  margin-bottom: 3px;
                  width: 120px;
                }
                .s_panel {
                  border-radius: 2px;
                  border: 1px solid #CCC;
                  background-color: #FBFDF2;  
                }
            </style> ", FALSE));
        $iids = $inventory->getiids();
        for($i = 0; $i < $iids->length(); $i++){
            $item = $inventory->getitem($iids[$i]);
                 
            # Descriptions of the item functions
            switch ($item->function) {
                case "Click1": $usage = "<br/><b>use:</b> Feed a pet to give them {$item->value} EXP."; break;
                case "Click2": $usage = "<br/><b>use:</b> Feed a pet to set their EXP to {$item->value}."; break;
                case "Click3": $usage = "<br/><b>use:</b> Resets EXP earned today to 0."; break;
                case "Level1": $usage = "<br/><b>use:</b> Raises the Level of your pet by {$item->value}."; break;
                case "Level2": $usage = "<br/><b>use:</b> Sets the Level of your pet to {$item->value}."; break;
                case "Level3": $usage = "<br/><b>use:</b> Makes your pet Level 0 again!"; break;
                case "Gender": $usage = "<br/><b>use:</b> Swaps the gender of your pet to its opposite!"; break;            
                default;
                    $usage = ""; break;
            }  # End item function descriptions        

            
            # Rendering items now
            $document->add(new Comment("
            <div class=\"s_panel sc_item\">
            <img rel=\"tooltip\" title=\"{$item->description} <em>{$usage}</em>\" src=\"{$item->imageurl}\"/><br/>
              <b>{$item->itemname}</b><br> Own &times;{$item->quantity}<br/>", FALSE));

            # If item is consumable, add use button
            if($item->consumable == "yes") {
                $useForm = new FormBuilder("useform", "inventory/uses", "post");
                $useForm->setLineBreak(FALSE);
                $useForm->buildPasswordField("hidden", "action", "uses")
                        ->buildPasswordField("hidden", "itemname", $item->itemname)
                        ->buildButton("Use", "use", "use");
                $document->add($useForm);
            }

            # Add sellback button so long as the item is not a key item
            $sellback = $item->price / 2;
            $document->add(new Comment("<hr>{$sellback} {$mysidia->settings->cost} Each ", FALSE));
            if($item->category != "Key Items") {
                $sellForm = new FormBuilder("sellform", "inventory/sell", "post");
                $sellForm->setLineBreak(FALSE);
                $sellForm->buildPasswordField("hidden", "action", "sell")
                         ->buildPasswordField("hidden", "itemname", $item->itemname);
                
                $quantity = new TextField("quantity");
                $quantity->setSize(3);
                $quantity->setMaxLength(3);
                $quantity->setLineBreak(FALSE);

                $sell = new Button("Sell", "sell", "sell");
                $sell->setLineBreak(FALSE);

                $sellForm->add($quantity);
                $sellForm->add($sell);
                            $document->add($sellForm);
            }    

            $document->add(new Comment("</div>", FALSE));   

        } # END item for loop

    } # END index function  
    
    public function sell(){
        $mysidia = Registry::get("mysidia");
        $document = $this->document;
        $document->setTitle($this->lang->global_transaction_complete);
        $document->addLangvar("{$this->lang->sell}{$mysidia->input->post("quantity")} {$mysidia->input->post("itemname")} {$this->lang->sell2}");
    }
    
    public function toss(){
        $mysidia = Registry::get("mysidia");
        $document = $this->document;
        if($mysidia->input->get("confirm")){
            $document->setTitle($this->lang->global_action_complete);
            $document->addLangvar("{$this->lang->toss}{$mysidia->input->post("itemname")}{$this->lang->toss2}");
            return;
        }
    
        $document->setTitle($this->lang->toss_confirm);
        $document->addLangvar($this->lang->toss_warning);    

        $confirmForm = new FormBuilder("confirmform", "toss/confirm", "post");
        $confirmForm->buildPasswordField("hidden", "action", "toss")
                    ->buildPasswordField("hidden", "itemname", $mysidia->input->post("itemname"))
                    ->buildButton("Please Toss", "confirm", "confirm");
        $document->add($confirmForm);            
    }
}
?>

Anyway, yeah, the Key item thing seems to be a bug or afterthought with the baseline Mysidia framework... by the sounds of it. In classes/class_itemtablehelper.php, the category "Key Items" is referenced twice, rather than the function "Key". I literally just imitated the existing file to render the form, so I copied over the same error.

However, if I do create an item with the category "Key Items" on a default install, it renders as expected on a default install of Mysidia (you can never sell key items). I cannot actually recreate your problem on a default install whatsoever.

  Spoiler: screenshot of default install with key items working as intended 

So, this DOES come down to being a bug with my mod, AND I managed to fix it. The fix is in the file above. (I moved $document->add($sellForm); inside the if statement, because that makes more sense, anyway. The variable should have still never been built to display, anyway... but apparently it was?)