So if you watch the video @21:45 I talk about how the “Update Buttons” function works. Since all the inventory buttons are default set to “Disabled” when you collect an item is the only time an inventory button may become “Enabled” hence why it is not “called” from the UI. It exists IN the UI but it is “called” from the master item when the player “collects” the item. It has a hard coded index value that is added to the collected items array, that collected items array is what tells the “Update Button” function which buttons should be enabled. So the error you are getting is because you are likely reading a null value for a button. Double check that the array actually contains something when you collect an item and then make sure all your items are properly indexed (that integer variable I created in the master BP MUST be changed in each CHILD BP, you can’t leave it at -1 this will cause a null value to be added to the collected item array) Let me know if any of those could be the potential issues.
There are no other widgets being made… The other code had nothing to do with widgets. it was for highlighting the model with a material using post process volume, which i dont want anymore as overlapping and destroying the item works just fine. The only widgets that exist in my game are HUD and Inventory…
I GOT IT WORKING! So my issue wasnt the child blueprints or even the widget buttons or anything. What the issue was is that I was creating my inventory widget only when I pressed the key. I had it being made on my character begin play but wasnt calling to the viewport and hiding it. Thats what my issue was because everytime I started the game you would have to open the inventory to get the buttons to even register on the construct. I changed my setup to the same FlipFlop method you had rather than my is inventory open method. Sorry for all this confusion and THANK YOU SO MUCH! for all the help! you are literally a time saver 
TO ALL WHO SEE THIS POST. The video that Nebula Games posted does work!!!
Here is the link in case you miss it:
Haha yea that will do it. Otherwise you are calling a function for a widget that doesn’t exist yet. You have to create it before you start collecting anything. Glad it’s working as expected now! Let me know if you need help with anything else.