I'm afraid that you don't have a Panel with the name 'Panneau' in your formHistory layout.
Initialize is needed if you add views by code but you must also add this view to a parent view.
Initialize is not needed when the view is defined in the layout file.
You should use this code if you want to use event from the panel:
Panneau.Initialize("Panneau")
Activity.AddView(Panneau, 0, 360, 300, 140)
And you could use this code if you don't want to use event from the panel:
Panneau.Initialize("")
Activity.AddView(Panneau, 0, 360, 300, 140)
To be able to give you a concrete answer we need to have your whole project as a zip file (IDE menu File/Export As zip.
Best regards.