Painting app crashes if invalid color entered

[cheapie]

1. Open the Painting app (on any computer)
2. Enter a color not consisting of valid hex digits (for example, a color entry of h or potato)
3. Click Set
4. Server crashes

Example output:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'laptop_extend_pack_2' in callback node_on_receive_fields(): ...creative/worldmods/minetest-laptop/apps/painting_app.lua:174: bad argument #5 to 'format' (number expected, got nil)
ERROR[Main]: stack traceback:
ERROR[Main]: 	[C]: in function 'format'
ERROR[Main]: 	...creative/worldmods/minetest-laptop/apps/painting_app.lua:174: in function <...creative/worldmods/minetest-laptop/apps/painting_app.lua:105>
ERROR[Main]: 	...dic/worlds/creative/worldmods/minetest-laptop/app_fw.lua:36: in function 'receive_data'
ERROR[Main]: 	...medic/worlds/creative/worldmods/minetest-laptop/mtos.lua:402: in function 'pass_to_app'
ERROR[Main]: 	...orlds/creative/worldmods/minetest-laptop/hardware_fw.lua:64: in function <...orlds/creative/worldmods/minetest-laptop/hardware_fw.lua:62>