Fix the use-after-free bug for previous_widget

ChangeLog

@@ -1,3 +1,6 @@
+2016-06-27  Markus Gans  <guru.mail@muenster.de>
+	* Fix the use after free bug for previous_widget
+
 2016-06-26  Markus Gans  <guru.mail@muenster.de>
 	* Bug fix for FWindow::activatePrevWindow()
 

doc/TODO

@@ -25,4 +25,6 @@ Missing Features
 - A possibility to change the window size dynamically with the mouse
 - Add a window switcher on the right side of the status bar
 - Add a scrolling area with on-demand scroll bars for FButtonGroup
+- Adding for flexible layouts a FGrid widget container that organizes
+  its child widgets in rows and columns
 

src/fwindow.cpp

@@ -22,7 +22,10 @@ FWindow::FWindow(FWidget* parent)
 
 //----------------------------------------------------------------------
 FWindow::~FWindow()  // destructor
-{ }
+{
+  if ( previous_widget == this )
+    previous_widget = 0;
+}
 
 
 // protected methods of FWindow

test/ui.cpp

@@ -740,7 +740,7 @@ void MyDialog::cb_view (FWidget*, void* data_ptr)
     return;
 
   FDialog* view = new FDialog(this);
-  view->setText ("viewer");
+  view->setText ("Viewer");
   view->setGeometry (1+int((getRootWidget()->getWidth()-60)/2),
                      int(getRootWidget()->getHeight()/6),
                      60,