diff --git a/modules/juce_gui_basics/native/juce_android_Windowing.cpp b/modules/juce_gui_basics/native/juce_android_Windowing.cpp
index 9fa4733777..369a991bd6 100644
--- a/modules/juce_gui_basics/native/juce_android_Windowing.cpp
+++ b/modules/juce_gui_basics/native/juce_android_Windowing.cpp
@@ -665,7 +665,10 @@ JUCE_JNI_CALLBACK (JUCE_ANDROID_ACTIVITY_CLASSNAME, alertDismissed, void, (JNIEn
                                                                            jlong callbackAsLong, jint result))
 {
     if (ModalComponentManager::Callback* callback = (ModalComponentManager::Callback*) callbackAsLong)
+    {
         callback->modalStateFinished (result);
+        delete callback;
+    }
 }
 
 //==============================================================================
diff --git a/modules/juce_gui_basics/native/juce_ios_Windowing.mm b/modules/juce_gui_basics/native/juce_ios_Windowing.mm
index 963179d0f4..fe06fbb40d 100644
--- a/modules/juce_gui_basics/native/juce_ios_Windowing.mm
+++ b/modules/juce_gui_basics/native/juce_ios_Windowing.mm
@@ -154,7 +154,7 @@ private:
     int result;
     JuceAlertBoxDelegate* delegate;
     UIAlertView* alert;
-    ModalComponentManager::Callback* callback;
+    ScopedPointer<ModalComponentManager::Callback> callback;
     const bool isYesNo, isAsync;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (iOSMessageBox)
diff --git a/modules/juce_gui_basics/native/juce_mac_Windowing.mm b/modules/juce_gui_basics/native/juce_mac_Windowing.mm
index 3f7afe8bfb..b629729d5a 100644
--- a/modules/juce_gui_basics/native/juce_mac_Windowing.mm
+++ b/modules/juce_gui_basics/native/juce_mac_Windowing.mm
@@ -67,7 +67,7 @@ public:
 private:
     AlertWindow::AlertIconType iconType;
     String title, message;
-    ModalComponentManager::Callback* callback;
+    ScopedPointer<ModalComponentManager::Callback> callback;
     const char* button1;
     const char* button2;
     const char* button3;
diff --git a/modules/juce_gui_basics/native/juce_win32_Windowing.cpp b/modules/juce_gui_basics/native/juce_win32_Windowing.cpp
index 5b148cfb69..0542d03e2d 100644
--- a/modules/juce_gui_basics/native/juce_win32_Windowing.cpp
+++ b/modules/juce_gui_basics/native/juce_win32_Windowing.cpp
@@ -2931,14 +2931,12 @@ class WindowsMessageBox  : public AsyncUpdater
 {
 public:
     WindowsMessageBox (AlertWindow::AlertIconType iconType,
-                       const String& title_, const String& message_,
-                       Component* associatedComponent,
-                       UINT extraFlags,
-                       ModalComponentManager::Callback* callback_,
-                       const bool runAsync)
+                       const String& boxTitle, const String& m,
+                       Component* associatedComponent, UINT extraFlags,
+                       ModalComponentManager::Callback* cb, const bool runAsync)
         : flags (extraFlags | getMessageBoxFlags (iconType)),
           owner (getWindowForMessageBox (associatedComponent)),
-          title (title_), message (message_), callback (callback_)
+          title (boxTitle), message (m), callback (cb)
     {
         if (runAsync)
             triggerAsyncUpdate();
@@ -2964,7 +2962,7 @@ private:
     UINT flags;
     HWND owner;
     String title, message;
-    ModalComponentManager::Callback* callback;
+    ScopedPointer<ModalComponentManager::Callback> callback;
 
     static UINT getMessageBoxFlags (AlertWindow::AlertIconType iconType) noexcept
     {