LanguageClient: Fix crash on closing qc while restarting a server

Reset the assist providers when restarting a server, so the client can
be safely shutdown while closing Qt Creator.

Change-Id: I4d5f18582a8c67cbe02974d72fcef9703ed64dc4
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/client.cpp

@@ -814,6 +814,8 @@ bool Client::reset()
     m_project = nullptr;
     for (const DocumentUri &uri : m_diagnostics.keys())
         removeDiagnostics(uri);
+    for (TextEditor::TextDocument *document : m_resetAssistProvider.keys())
+        resetAssistProviders(document);
     return true;
 }