fix: compatible with frappe dialog email when pre-fill content and get newest content.

[anhlevfx]

When create a new email, frappe will popup a dialog, if we access form doctype in here will throw undefined exception. Check undefined is necessary. Beside, send email action will call function get_input_value, the result will always empty because it use the function in class ControlCode. So, the extend class must override it.

[shridarpatil]

can you add a GIF, this will help me understand the the fixes

[anhlevfx]

Sorry, recently I'm too busy for gif record. The fix I submitted for two issues:

1. Can't open email popup like this
2. Content email will empty.

[mohsinalimat]

Email template not appear when select Email Template from Email PopUp

[mohsinalimat]

https://github.com/shridarpatil/frappe_tinymce/issues/7