Some of my materials disappear after I run the app for a few minutes and then I get an unhandled exception

I have been trying to bring images from outside the unreal project to make them textures to the put inside of an already established material. I successfully brought them into the project and they display for a time but then they disappear and then this error comes up:

Unhandled exception at 0x000007FED636F901 (UE4Editor-Engine-Win64-Debug.dll) in UE4Editor-Win64-Debug.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.  

I know this has something to do with me switching out the texture of a material with an outside source but I’m trying to go back in the stack to see where the program found a null. I don’t understand where the stack is in the program or how it is relevant to the material.
The the last element in the stack leads me to the StaticMeshRender.cpp line 486 (const FLODInfo& ProxyLODInfo = LODs[LODIndex]:wink:

Could someone possibly explain to me what the program is doing and how I can temporarily change the texture or temporarily and pragmatically import something from the outside and delete it when the application closes?

I tried it again and this time the error came from a different part of the code
the IsA() method had an error which is located in UObjectBaseUtility.cpp. The line where the error accured is line 257 (for ( UClass* TempClass=GetClass(); TempClass; TempClass=TempClass->GetSuperClass() ) ) It had the same error as I mentioned above

I found a .udn file called CustomImporters. Would I be able to use this to create my own importer?

I found the asset importer code for unreal and used that to import my files at run time. I haven’t had any issues since then.