Don't assert if CPluginManagerSimulator::createListener returns nullptr

CPluginManagerSimulator::createListener returns nullptr if no
listener could be created, e.g. because the plugin could not be loaded
due to missing dependencies. Handle this scenario properly instead of
raising an assert.

refs #669

src/blackcore/context/contextsimulatorimpl.cpp

@@ -419,7 +419,7 @@ namespace BlackCore
             }
 
             ISimulatorListener *listener = m_plugins->createListener(simulatorInfo.getIdentifier());
-            Q_ASSERT_X(listener, Q_FUNC_INFO, "No listener");
+            if (!listener) { return; }
 
             if (listener->thread() != &m_listenersThread)
             {

src/blackcore/pluginmanagersimulator.cpp

@@ -11,6 +11,7 @@
 #include "blackcore/pluginmanagersimulator.h"
 #include "blackcore/simulator.h"
 #include "blackmisc/directoryutils.h"
+#include "blackmisc/logmessage.h"
 
 #include <QJsonObject>
 #include <QJsonValue>
@@ -42,9 +43,10 @@ namespace BlackCore
         PluginExtended &plugin = m_plugins[pluginId];
         if (!plugin.listener)
         {
-            ISimulatorFactory *factory = getPluginById<ISimulatorFactory>(pluginId);
+            ISimulatorFactory *factory = getFactory(pluginId);
             if (!factory)
             {
+                CLogMessage(this).warning("Could not load plugin %1.") << pluginId;
                 m_plugins.remove(pluginId);
                 return nullptr;
             }

src/blackcore/pluginmanagersimulator.h

@@ -46,6 +46,7 @@ namespace BlackCore
 
         //! Create simulator listener from the plugin
         //! In case one is existing already, it is returned instead.
+        //! Returns nullptr if no listener could be created.
         ISimulatorListener *createListener(const QString &pluginId);
 
         //! Get previously created simulator listener from the plugin