I was trying to add custom icons (.png 64*64) in Query widget in WAB developer edition. I have to add every time, after some time they disappear frequently. I don't know what is wrong with them. Is it a bug or something else. In the developer tool, I got the error "Failed to load resource: the server responded with a status of 404 (Not Found) icon_1.png". I have added a screen shot for more detail.