Installation does not finish because of non-modal message box hidden by InstallShield dialog

After finishing the installation process the installation pops up a message box that tells the user that he can now plug in the hardware safely. The message box does not have the attribute topmost set. Consequently it is sometimes hidden by the "Finish Installation" dialog. As long as the message box is not closed the "Finish" button of the installation dialog is not enabled and the installation cannot be terminated in a regular fashion.