1.
After your app exits its applicationDidEnterBackground: method, it must not make any new OpenGL ES calls. If it makes an OpenGL ES call, it is terminated by iOS.
https://developer.apple.com/library/content/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/ImplementingaMultitasking-awareOpenGLESApplication/ImplementingaMultitasking-awareOpenGLESApplication.html#//apple_ref/doc/uid/TP40008793-CH5-SW1
http://stackoverflow.com/questions/12926495/opengl-es-application-crashes-when-locked-screen-or-entered-background