Handle changes to CLIENT_FRAME_CHANGED in lag display plugin

Fixes issue 3459

Change-Id: I0f0a177b065b695704e1f39595c020be63b98992
Reviewed-on: http://gerrit.dmdirc.com/404
Reviewed-by: Gregory Holmes <greboid@dmdirc.com>
Tested-by: Gregory Holmes <greboid@dmdirc.com>

src/com/dmdirc/addons/lagdisplay/LagDisplayPlugin.java

@@ -194,7 +194,7 @@ public final class LagDisplayPlugin extends Plugin implements ActionListener, Co
             panel.refreshDialog();
         } else if (type.equals(CoreActionType.CLIENT_FRAME_CHANGED)) {
             final FrameContainer source = (FrameContainer) arguments[0];
-            if (source.getServer() == null) {
+            if (source == null || source.getServer() == null) {
                 panel.setText("Unknown");
             } else if (source.getServer().getState() != ServerState.CONNECTED) {
                 panel.setText("Not connected");