Reset most_recently_raised when removing ShellClient

Summary:
This time without a test case that could trigger a crash as the only
reading usage of the variable could luckily not crash.

Reviewers: #plasma, #kwin

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6853

workspace.cpp

@@ -425,6 +425,9 @@ void Workspace::init()
         connect(w, &WaylandServer::shellClientRemoved, this,
             [this] (ShellClient *c) {
                 m_allClients.removeAll(c);
+                if (c == most_recently_raised) {
+                    most_recently_raised = nullptr;
+                }
                 if (c == delayfocus_client) {
                     cancelDelayFocus();
                 }