NavigationBar: Clear current webview for all tool buttons

Fixes another crash similar to #2537

src/lib/navigation/navigationbar.cpp

@@ -189,6 +189,11 @@ NavigationBar::NavigationBar(BrowserWindow* window)
     loadSettings();
 }
 
+NavigationBar::~NavigationBar()
+{
+    setCurrentView(nullptr);
+}
+
 void NavigationBar::setSplitterSizes(int locationBar, int websearchBar)
 {
     QList<int> sizes;

src/lib/navigation/navigationbar.h

@@ -43,6 +43,7 @@ class FALKON_EXPORT NavigationBar : public QWidget
 
 public:
     explicit NavigationBar(BrowserWindow* window);
+    ~NavigationBar();
 
     void setSplitterSizes(int locationBar, int websearchBar);