How to use bpy.path.abspath('//') as default for StringProperty?

I would like to have a StringProperty of subtype DIR_PATH initialized to a default of bpy.path.abspath('//'). The property is on an operator with a poll() method that checks to ensure the Blender file has been saved first. This stops working if my add-on is installed, Blender throws a AttributeError: '_RestrictData' object has no attribute 'filepath' error. I am aware that _RestrictData is an artifact of the class registration process however I have no idea how to work around it in this case.

bl_info = {
    "name": "Example to show abspath error",
    "version": (0, 0, 1),
    "blender": (3, 0, 0),
    "category": "Import-Export",
    "location": "View3D > UI > MyCompany",
    "description": "Sample code for Stack Exchange to illustrate problem with abspath and a StringProperty default value"
}

import bpy

class MYCOMPANY_OT_DummyOp(bpy.types.Operator):
    bl_label="Dummy class to show error"
    bl_idname = "mycompany.dummyop"
    ExportFolder: bpy.props.StringProperty(name="Export dir", description="Directory to write out exported file", default=bpy.path.abspath('//'), subtype='DIR_PATH')

    @classmethod
    def poll(cls, C):
        if not bpy.data.filepath:
            cls.poll_message_set("Current .BLEND file hasn't been saved, this is necessary for operation")
            return False
        return True

    def execute(self, C):
        return {'FINISHED'}


class MYCOMPANY_PT_TestPanel(bpy.types.Panel):
    bl_label="Test Panel"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"

    def draw(self, context):
        L=self.layout
        L.operator("mycompany.dummyop")


classesToRegister = (
    MYCOMPANY_OT_DummyOp,
    MYCOMPANY_PT_TestPanel
    )

def register():
    for c in classesToRegister:
        bpy.utils.register_class(c)

def unregister():
    for c in reversed(classesToRegister):
        bpy.utils.unregister_class(c)

if __name__ == "__main__":
    register()