Added an assertion that will be triggered at the moment you attempt to delete a LookAndFeel object that's still in use somewhere

examples/Demo/Source/MainWindow.cpp

@@ -126,6 +126,12 @@ public:
         addAndMakeVisible (demoList);
     }
 
+    ~ContentComponent()
+    {
+        // before deleting our lookandfeel object, make sure it's no longer in use
+        LookAndFeel::setDefaultLookAndFeel (nullptr);
+    }
+
     void clearCurrentDemo()
     {
         currentDemo = nullptr;

modules/juce_gui_basics/lookandfeel/juce_LookAndFeel.cpp

@@ -50,6 +50,16 @@ LookAndFeel::LookAndFeel()
 
 LookAndFeel::~LookAndFeel()
 {
+    /* This assertion is triggered if you try to delete a LookAndFeel object while it's
+         - still being used as the default LookAndFeel; or
+         - is set as a Component's current lookandfeel; or
+         - pointed to by a WeakReference somewhere else in the code
+
+       Deleting a LookAndFeel is unlikely to cause a crash since most things will use a
+       safe WeakReference to it, but it could cause some unexpected graphical behaviour,
+       so it's advisable to clear up any references before destroying them!
+    */
+    jassert (masterReference.getNumActiveWeakReferences() == 0);
 }
 
 //==============================================================================