fix crash when closing rssguard with opened log dialog

resources/desktop/io.github.martinrotter.rssguard.metainfo.xml

@@ -60,7 +60,7 @@
   <content_rating type="oars-1.0" />
   <content_rating type="oars-1.1" />
   <releases>
-    <release version="4.2.5" date="2022-12-02" />
+    <release version="4.2.5" date="2022-12-06" />
   </releases>
   <provides>
     <binary>rssguard</binary>

src/librssguard/miscellaneous/application.cpp

@@ -215,6 +215,9 @@ Application::~Application() {
   }
 #endif
 
+  // Disable logging into the form.
+  m_logForm = nullptr;
+
   qDebugNN << LOGSEC_CORE << "Destroying Application instance.";
 }