When you try to release scene resources by calling method Assimp.aiReleaseImport it fails throwing NullPointerException:
java.lang.NullPointerException
at org.lwjgl.system.Checks.check(Checks.java:94)
at org.lwjgl.assimp.AIScene.validate(AIScene.java:439)
at org.lwjgl.assimp.Assimp.naiReleaseImport(Assimp.java:2560)
at org.lwjgl.assimp.Assimp.aiReleaseImport(Assimp.java:2572)
the reason is that method AIScene.validate(AIScene.java:439) requires a pointer to animation buffer while the scene may not contain animation.
Thanks, will be fixed in the first 3.1.2 nightly build. In the meantime, you can use:
JNI.invokePV(Assimp.Functions.ReleaseImport, scene.address());
Also I want to draw your attention, that AIScene may not contain information about textures, lights, cameras.
I think the easiest way - is to ignore the check of mandatory fields.
Yes, those fields have been fixed (https://github.com/LWJGL/lwjgl3/commit/6cb0b271f7c0dcdae97a2a49c5b753fab35644f8) as well.
The latest nightly (LWJGL 3.1.2 #1) includes the above.