a11y: assert on switching hatch/bitmap area radio button tab things

Change-Id: I2525123dfe2c2dedcbf808f5d2f8e054e4c4bdf1
Reviewed-on: https://gerrit.libreoffice.org/39690Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

svx/source/accessibility/svxpixelctlaccessiblecontext.cxx

@@ -355,7 +355,7 @@ void SAL_CALL SvxPixelCtlAccessible::addAccessibleEventListener( const uno::Refe
     {
         ::osl::MutexGuard   aGuard( m_aMutex );
         if (!mnClientId)
-                mnClientId = comphelper::AccessibleEventNotifier::registerClient( );
+            mnClientId = comphelper::AccessibleEventNotifier::registerClient( );
         comphelper::AccessibleEventNotifier::addEventListener( mnClientId, xListener );
     }
 }
@@ -365,7 +365,8 @@ void SAL_CALL SvxPixelCtlAccessible::removeAccessibleEventListener( const uno::R
     if (xListener.is())
     {
         ::osl::MutexGuard   aGuard( m_aMutex );
-
+        if (!mnClientId)
+            return;
         sal_Int32 nListenerCount = comphelper::AccessibleEventNotifier::removeEventListener( mnClientId, xListener );
         if ( !nListenerCount )
         {