Description of the Issue

When a plugin is using NPPM_CREATESCINTILLAHANDLE to get a valid buffer
and NPPM_DESTROYSCINTILLAHANDLE to destroy it, every next action which would request
a new buffer leads to a crash

Steps to Reproduce the Issue

1. send NPPM_CREATESCINTILLAHANDLE
2. send NPPM_DESTROYSCINTILLAHANDLE
3. open a new empty document

Expected Behavior

no crash

Actual Behavior

crash

Debug Information

Notepad++ v7.5.6 (64-bit)
Build time : Mar 19 2018 - 00:23:17
Path : D:\Tests\npp\756_64_with_pluginmanager\notepad++.exe
Admin mode : OFF
Local Conf mode : ON
OS : Windows 7 (64-bit)
Plugins : PluginManager.dll PythonScript.dll

Simple python script to demonstrate it

sci = notepad.createScintilla()
notepad.destroyScintilla(sci)
notepad.new()