BedWars | A new BedWars plugin for PocketMine-MP by XenialDan | Game Engine library

I think the error appears because you have not set a title for your inventory. To fix it, you should assign a value to inventory_name:

Premise. Adjusting the font size of a label to ensure that it fits the width of its container is usually not a good idea, most importantly because having different font sizes for the same widgets makes the whole interface incoherent. Don't underestimate this aspect, as it's very important for the user experience.

Also, as already suggested in the comments, using fixed geometries is usually a very bad idea. While I can understand the specific case of a game interface, this doesn't prevent to use layout managers for the container widgets. The main reason for this is that what you see on your computer (or even in Designer) is rarely what the users will actually see on their screens, and if the user resizes the window (possibly to maximize it or even resize to fit a smaller screen) you'll end up in an ugly or even not usable interface.
So, I strongly suggest you to use a layout manager for the whole interface, and use fixed geometries only for certain child widgets (only the labels, probably).

In any case, a possible solution is to use a custom class that inherits from QLabel and promote each label in designer to that custom class (more on this at the end of this post).

Here is a possible implementation of a custom class:

Instead of doing:

Please see Json.NET's Modifying JSON.

This sample loads JSON, modifies JObject and JArray instances and then writes the JSON back out again.

Sample

Now the issue is just updating the text-value of your label. To do so, you don't have to call iron.pack() repeatedly but just once (e.g. in your case before your main). Within your function update the text-property of the iron-label iron["text"] = i

Also you don't need to call root.mainloop() multiple times.

Try adding margin: 0 auto; to the .header-content

This will force the margin on either side of your container to fill the remaining space equally, resulting in a centered element.

I modified your fiddle: https://jsfiddle.net/pednvrf3/3/