Using Systemcall SCREEN_IS_KEY_PRESSED causes crashes at randon

[ParkerTenBroeck]

When using the SCREEN_IS_KEY_PRESSED system call this error can happen

[Error] [RunTime] [System Call] System Call: SCREEN_IS_KEY_PRESSED from plugin: Screen
java.lang.NullPointerException
        at org.parker.mips.plugin.internal.syscall.Screen.isKeyPressed(Screen.java:85)
        at org.parker.mips.plugin.internal.syscall.ScreenSystemCalls$1.handleSystemCall(ScreenSystemCalls.java:26)
        at org.parker.mips.plugin.syscall.SystemCallPluginHandler.SystemCall(SystemCallPluginHandler.java:110)
        at org.parker.mips.processor.InstructionDecode$4.runInstruction(InstructionDecode.java:62)
        at org.parker.mips.processor.InstructionDecode.runInstruction(InstructionDecode.java:598)
        at org.parker.mips.processor.Processor.singleStep(Processor.java:116)
        at org.parker.mips.processor.Processor.run(Processor.java:70)
        at java.base/java.lang.Thread.run(Thread.java:834)