You can't check if a blueprint is valid in a packaged build. They are an editor only time concept, how or why this worked before, I do not know, but the fact that the asset doesn't exist at runtime now is the correct behavior.
When using the IsValid node inside of a Widget blueprint with the Widget as the selected asset, the string will print as true in the Editor, but prints as false in the packaged build of the game.
This issue does NOT occur in 4.14.3 binary build of the engine but does occur in 4.15.2 and 4.16 preview 3 of the Binary builds of the engine
Head over to the existing Questions & Answers thread and let us know what's up.