How to fix instant lock “unfortunately com.android.systemui has stopped” on Moto G with ART

I have had this problem for a couple of days on my first-generation Motorola Moto G. Normally a restart would fix it. However now I am faced with the following error message when trying to unlock the phone after a restart:

Unfortunately the process com.android.systemui has stopped.

The phone has reset the background to the default home screen which disappears when I unlock, going to a black screen. However, it still shows my apps and widgets. Then a second or so later goes back to the unlock screen.

I have tried clearing the cache partition multiple times which has not worked. Ss there any way I can fix this problem without a factory reset, or taking my phone to a phone shop?

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Some additional information:

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  I have been using ART on my phone for the past couple of months, but the problem has only occurred recently

  
  


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  On boot-up I get the following message:

  
  

  
  Updating Android. Optimizing app 1 of 1.