Help with Fatal Exeption

Hello everybody,

I'm making an app at the moment, but I've got a little problem: 90% of the times I run the app, everything works fine. But sometimes, I get this error by starting the app:

FATAL EXCEPTION: Animation Thread
Process: processing.test.touchscreen, PID: 23581
java.lang.NullPointerException
    at processing.core.PGraphicsAndroid2D.backgroundImpl(Unknown Source)
    at processing.core.PGraphics.backgroundFromCalc(Unknown Source)
    at processing.core.PGraphics.background(Unknown Source)
    at processing.core.PGraphics.defaultSettings(Unknown Source)
    at processing.core.PGraphics.checkSettings(Unknown Source)
    at processing.core.PGraphicsAndroid2D.beginDraw(Unknown Source)
    at processing.core.PApplet.handleDraw(Unknown Source)
    at processing.core.PGraphicsAndroid2D.requestDraw(Unknown Source)
    at processing.core.PApplet.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:841)

Does anyone knows what this means? I'm not using PGraphics, so I don't understand why it comes with an PGraphics error!

Regards, Daantje