In Unity, there is a bug where some assets are not included in the build in some projects.
And in v3, this sometimes happens with the marker texture.
I don't know what the reason is. Maybe the asset id of texture matches something internal that shouldn't be included in the build.

How to fix it:
1. Find the marker texture in the project.
2. Make a copy of it (CTRL + C, CTRL + V).
3. Apply the import settings from the original texture to it.
4. Specify the new texture instead of the old one in Marker Manager.

That's it. You have the same texture with the same settings, but it will be included in the build.

Marker texture is null
UnityEngine.DebugLogHandler:Internal_Log(LogType, LogOption, String, Object)
OnlineMaps.MarkerFlatDrawer:DrawMarker(Marker2D, Map, Single)
OnlineMaps.MarkerFlatDrawer:OnDrawMarkers()
OnlineMaps.ControlBaseDynamicMesh:UpdateControl()
OnlineMaps.TileSetControl:UpdateControl()
OnlineMaps.Map:UpdateControl()
OnlineMaps.Map:CheckBufferComplete()
OnlineMaps.Map:LateUpdate()

Not sure why I don't see this error while testing in Unity, but the example doesn't seem to do anything.  So then I tried using the  example on iOS, and that's when I get the error.

Sorry for the extra work.  Just curious if you can point me in the correct direction.  BTW, I don't see any FIX buttons in the inspector.

Thanks.