Unable to process powershell function with content from windows form

function CalendarShare {
    Add-MailboxFolderPermission -Identity ${FromUser.Text} -AccessRights Editor -User ${ToUser.Text}
}

When the program is running, it works until it processes the share calendar button. It states that it

cannot bind the argument parameter identity because it is null.

I have no idea what can cause this

Full code:

Add-Type -AssemblyNAme System.Windows.Forms
[System.Windows.Forms.Application]::EnableVisualStyles()

#Declare Functions

function login {
    $LiveCred = Get-Credential
    $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
    Import-PSSession $Session
    $btnlogin.Visible = $False
}
function quitprogram {
    $Form.Close()
    Exit-PSSession []
}

function CalendarShare {
    Add-MailboxFolderPermission -Identity ${FromUser.Text} -AccessRights Editor -User ${ToUser.Text}
}

#Creates base form
$Form = New-Object system.Windows.Forms.Form
$Form.ClientSize = '400,400'
$Form.Text = 'Powershell GUI'
$Form.TopMost = $False

#Creates Login button
$btnlogin = New-Object System.windows.Forms.button
$btnlogin.text = 'Login'
$btnlogin.width = 80
$btnlogin.height = 40
$btnlogin.location = New-Object System.Drawing.Point(150,5)
#Add_Click defines what to do on click
$btnlogin.Add_Click( {
    login #function previously defined. Line 6
})

#Creates label for From user
$FromUserLabel = New-Object system.Windows.Forms.Label
$FromUserLabel.text = 'Share Calendar From User'
$FromUserLabel.AutoSize = $true
$FromUserLabel.width = 25
$FromUserLabel.height = 10
$FromUserLabel.location = New-Object System.Drawing.Point(130,100) #Define where it appears on the map

#Create first input. From User
$FromUser = New-Object system.Windows.Forms.TextBox
$FromUser.multiline = $False
$FromUser.width - 200
$FromUser.height = 20
$FromUser.location = New-Object System.Drawing.Point(135,125)

#Create label for To user
$ToUserLabel = New-Object system.Windows.Forms.Label
$ToUserLabel.text = 'Share Calendar To User'
$ToUserLabel.AutoSize = $True
$ToUserLabel.width = 25
$ToUserLabel.height = 10
$ToUserLabel.location = New-Object System.Drawing.Point(135,175)

#Create second input. To User
$ToUser = New-Object system.Windows.Forms.TextBox
$ToUser.multiline = $False
$ToUser.width - 200
$ToUser.height = 20
$ToUser.location = New-Object System.Drawing.Point(135,200)

#Create Share Button Calendar
$btnsharecalendar = New-Object System.windows.Forms.button
$btnsharecalendar.text = 'Share Calendar'
$btnsharecalendar.width = 165
$btnsharecalendar.height = 30
$btnsharecalendar.location = New-Object System.Drawing.Point(105,275)
#Define Add_Click below
$btnsharecalendar.Add_Click({
    CalendarShare
})



#Create button to close program
$btnquit = New-Object System.windows.Forms.button
$btnquit.text = 'Quit'
$btnquit.width = 80
$btnquit.height = 40
$btnquit.location = New-Object System.Drawing.Point(150,325)
#Add_Click defines what to do on click
$btnquit.Add_Click( {
    quitprogram #function previously defined. Line 12
})


#Allows form to work
$Form.Controls.AddRange(@($btnlogin,$FromUserLabel,$FromUser,$ToUserLabel,$ToUser,$btnsharecalendar,$btnquit))
$Form.ShowDialog()

Note: This is assuming that both $FromUser and $ToUser are textboxes

You aren't using your variable correctly for the identity parameter, should be:

function CalendarShare {
    Add-MailboxFolderPermission -Identity $FromUser.Text -AccessRights Editor -User $ToUser.Text}
}