ios error : Assertion failed: (_colorByNumber.size() > 0)

Sorry if this is an easy answer, but I am a real newbe in UE4

I am actually packaging a project for IOS, project made by someone else, and I can’t access him for the moment.

one map has an issue : a display which works properly in the editor, and as a mac os app, does not function.

The error code is the following :

LogPlayLevel: env: 2017-04-12 15:10:20.600 instruments[13964:31604] WebKit Threading Violation - initial use of WebKit from a secondary thread.
LogPlayLevel: env: Assertion failed: (_colorByNumber.size() > 0), function -[XRTheme _prepareColorsForLegacyUse], file /Library/Caches/, line 568.
LogPlayLevel: Deleting /Users/serge/Documents/deva/devaUE/Saved/StagedBuilds/IOS/launch.trace
LogPlayLevel: InternalUtils.SafeFindFiles: SafeFindFiles /var/folders/t1/gr8z_r4147q20cnlgzzh_m0c0000gn/T/+Users+Shared+Epic+Games+UE_4.8/Logs * False
LogPlayLevel: InternalUtils.FindFiles: FindFiles /var/folders/t1/gr8z_r4147q20cnlgzzh_m0c0000gn/T/+Users+Shared+Epic+Games+UE_4.8/Logs * False
LogPlayLevel: BuildCommand.Execute: ERROR: BUILD FAILED

What is it that I should do to this particular map ?

Thx for any answer !