TextEditor: Destroy completion widget if editor is closed

Task-number: QTCREATORBUG-14991
Change-Id: I7a23d4bd3a9fd97838f21e964a17b091dc925175
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/texteditor/codeassist/codeassistant.cpp

@@ -550,6 +550,7 @@ CodeAssistant::CodeAssistant() : d(new CodeAssistantPrivate(this))
 
 CodeAssistant::~CodeAssistant()
 {
+    destroyContext();
     delete d;
 }