Unable to add more customers to a project

When 'Allow Multiple Projects' is enabled, it can only be possible to add new customers to a project, provided the project has NEVER had any drafts containing any transaction lines associated with it. This means not only Open drafts, but also Processed drafts where the invoice has already been created. However a project which once contained a draft that has only had allocation lines written off in it, can still have customers added to it, for example.
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It's possible to remove a customer from a project, so long as there are no Open drafts. Customers can however still be removed from projects when drafts associated with the project have been previously Processed.