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The problem is there is no tile in the array at the i-th slot. You need to assign tiles to the array before you can set their colors.

Any time before you want to access it.

Is the value of tiles[i] null?
If so, where do you assign it a value?
You have defined an array, but you have not put any objects into the array. All its slots are empty.
You must assign a value...

Please copy and paste here the full text of the error message.
Look at the line number referenced in your program and find the variable that has a null value. Then backtrack in the code to see why...