This warning occurs when you have a lot of GDI objects running. You can close MultiCharts and all of its processes (open Task Manager->Processes tab) and then start MultiCharts again. You can also track the amount of GDI objects you are using by going to Task Manager->Processes Tab->View->Select Columns…->GDI Objects.
It is important to know that GDI objects are all objects that form the MultiCharts window and charts in it (all toolbars, menus, buttons, bars, drawings, etc). The Windows system limitation per one process is 10,000 objects. For stability reasons MultiCharts has put a limit of 7,500 GDI objects per MultiCharts process. In order to avoid such issue in the future please run multiple instances of MultiCharts application. To do that, please double click on MultiCharts icon on your desktop or in the start menu. Each instance will be able to handle up to 7,500 objects. You can utilize the new Desktops functionality to setup each instance the way you need it.