GWT Designer+maven in Eclipse = error "GWT widgets can be used only in client package of GWT module"

In Eclipse i imported project with "import ->existing maven projects".
I tried to use GWT Designer and got error "GWT widgets can be used
only in client package of GWT module." I can make new GWT Java
Project, where GWT Designer work OK.
What i have do that my existing maven projects work with GWT Designer?
i think i have to some settings in project, but what?

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