This error has been popping up a lately since I added a function to a blueprint library that I call from a UMG graph, that has a cast to a UI object. It’s only been happening in this scenario. As I was taking screenshots for this post the error went away and I was able to save the project. I still want to document the error here though and if I can provide more info I will.
I also had this error with Blueprint Function Libraries. I stopped using those.
I reported it many times in the crash feedback.
I’m having the same issue.
How can this be resolved?
So I haven’t run into this issue in a while. But anytime it did happen the problem almost seemed to just fix it self. I think is correct in the fact it has something to do with blueprint function libraries.
Hi everyone,
Do you have any steps I can take to reproduce this on my end? Thank you.
Hi everyone,
We haven’t heard from you in quite some time. I am marking this thread as answered for tracking purposes. If you are still experiencing this error, please comment with the requested information.
I am still experiencing the problem. It’s connected to using blueprint libraries. I just don’t use them anymore (as of last night)
Do you have steps that I can take to reproduce this error on my end?
Nope. It’s so random. Sometimes the error pops up. Then randomly it will go away. 
Sorry.
But I will say it seems pretty common, I mean if you have a project, create a simple function library, call it from different parts of the app, and just wait because it will peak it’s ugly head. It’s happened on two different machines (one with Windows 7 and 1 with Windows 10)
I haven’t been able to reproduce this on my end thus far. I will keep trying. In the mean time, can you show me a screenshot of a particular setup that has caused this error for you or if you do think of some repro steps I would be happy to see them and attempt to reproduce this on my end.
I was going to respond over the weekend with screenshots and this and that, but I completely removed all aspects of the blueprint library calls and to properly show you I’d have to recreate that. Just close the topic for now, and if I run into this problem anymore in the future I will document it better and make a new post. Thanks!
Hey 
Hope this helps
I tried to give access to as much details as possible.
I’m having this right now and it’s preventing me from getting anything saved.
In my case, it was creating a Boolean variable in the Persistent(Level BluePrint) which I set in order to affect a Widget. Now it won’t let me save.
Thing is, it also puts me at a loss as to how to trigger an event based on a played cinematic. I’m going to keep looking, but if you have an idea, I’d really die to hear it.
Nice find! Like I mentioned, my error was rather intermittent so maybe if you’re persistent you’ll get it saved in a little bit 
try adding a cast node between your “get all actors of class” and your branch. It looks like it is registering a null value because it has not confirmed that the variable is being pulled from the blueprint you are calling with your “get all actors of class”.