check that HWNDs are non-NULL before doing anything with them (part of patch 1866053)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51077 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/subwin.h

@@ -40,6 +40,7 @@ public:
     {
         for ( size_t n = 0; n < m_count; n++ )
         {
+            if ( m_hwnds[n] )
                 ::DestroyWindow(m_hwnds[n]);
         }
 
@@ -95,6 +96,7 @@ public:
         int sw = show ? SW_SHOW : SW_HIDE;
         for ( size_t n = 0; n < m_count; n++ )
         {
+            if ( m_hwnds[n] )
                 ::ShowWindow(m_hwnds[n], sw);
         }
     }
@@ -104,6 +106,7 @@ public:
     {
         for ( size_t n = 0; n < m_count; n++ )
         {
+            if ( m_hwnds[n] )
                 ::EnableWindow(m_hwnds[n], enable);
         }
     }
@@ -115,6 +118,8 @@ public:
         wxCHECK_RET( hfont, _T("invalid font") );
 
         for ( size_t n = 0; n < m_count; n++ )
+        {
+            if ( m_hwnds[n] )
             {
                 ::SendMessage(m_hwnds[n], WM_SETFONT, (WPARAM)hfont, 0);
 
@@ -122,18 +127,23 @@ public:
                 ::InvalidateRect(m_hwnds[n], NULL, FALSE /* don't erase bg */);
             }
         }
+    }
 
     // find the bounding box for all windows
     wxRect GetBoundingBox() const
     {
         wxRect r;
         for ( size_t n = 0; n < m_count; n++ )
+        {
+            if ( m_hwnds[n] )
             {
                 RECT rc;
+
                 ::GetWindowRect(m_hwnds[n], &rc);
 
                 r.Union(wxRectFromRECT(rc));
             }
+        }
 
         return r;
     }