poll failed - context is incorrect

Question

Goal

Generate several scene from files (csv, yml with parameters) and thus, initialize the scene before loading a file.

Bug

In my code, I am creating a plane(object mode) and want to subdivide it (edit mode). Yet I have this error message:

RuntimeError: Operator bpy.ops.object.editmode_toggle.poll() failed, context is incorrect

can anyone has a solution?

Info: before toggle to edit mode, I select and activate the plane and on my Blender GUI, the plane seems to be in Edit mode (with the vertices visible) yet I am still in object mode 😕

UPDATE

It seems that it come from the bpy.ops.wm.read_homefile()..

@rob: I made a little addons that you can place in a directory "test_addons" in the scripts/addons. This show how you cannot (?) use

bpy.ops.wm.read_homefile()

with a toggle_mode right after...

__ init__.py

bl_info = {
    "name": "Test Addons",
    "author": "",
    "location": "View3D > Tools > Test_addons",
    "version": (1, 0, 0),
    "blender": (2, 7, 9),
    "description": " ",
    "wiki_url": "",
    "category": "Development"
}


if "bpy" in locals():
    import importlib
    importlib.reload(ui)    
    importlib.reload(new_plane) 

else:
    import bpy
    from bpy.types import PropertyGroup
    from bpy.props import (PointerProperty, FloatProperty)
    from . import ui
    from . import new_plane 


class MyPlane(PropertyGroup): 

    size = FloatProperty(name="Size", default=5., min=0, max=100,)

classes = (ui.NewToolBar, new_plane.NewPlane, MyPlane)

def register():
    for cls in classes:     
        bpy.utils.register_class(cls)
    bpy.types.Scene.my_plane = PointerProperty(type=MyPlane)

ui.py

import bpy
from bpy.types import Panel


class NewToolBar(Panel):
    bl_category = "test_addons"
    bl_label = "test_addons"
    bl_region_type = "TOOLS"
    bl_space_type = "VIEW_3D"
    bl_context = "objectmode"

    def draw(self, context):                
        self.layout.operator("my_plane.new", text="New Plane")

new_plane.py

import bpy
from bpy.types import Operator

class NewPlane(Operator): 
    bl_idname = "my_plane.new"
    bl_label = "My Plane"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context): 

        # I load the home file
        bpy.ops.wm.read_homefile()

        # create an object and change its mesh in EDIT mode : I have an error message...
        my_plane = bpy.context.scene.my_plane
        bpy.ops.mesh.primitive_plane_add(radius=my_plane.size, location=(0., 0., 0.))
        bpy.ops.object.mode_set(mode='EDIT')
        bpy.ops.mesh.subdivide()
        bpy.ops.object.mode_set(mode='OBJECT')  

        return {'FINISHED'}

Error message

bpy.ops.object.mode_set(mode='EDIT')
File "C:\Blender\2.79\scripts\modules\bpy\ops.py", line 189, in __call__
ret = op_call(self.idname_py(), None, kw)
RuntimeError: Operator bpy.ops.object.mode_set.poll() failed, context is incorrect