V503-022 Prevent access to dangling pointers

Prevent access to dangling pointers which could occur when
the Buffer_View might want to refresh itself whilst the MDI
is being destroyed.

Change-Id: I61e027e8364785beecc8e0613aa1895647da9a4d

testsuite/tests/V503-022.buffer_view_finalization/a.adb

@@ -0,0 +1 @@
+

testsuite/tests/V503-022.buffer_view_finalization/p.gpr

@@ -0,0 +1,2 @@
+project p is
+end p;

testsuite/tests/V503-022.buffer_view_finalization/test.cmd

@@ -0,0 +1,2 @@
+cp perspectives6.xml $GNATSTUDIO_HOME/.gnatstudio/
+$GNATSTUDIO -Pp a.adb --load=python:test.py

testsuite/tests/V503-022.buffer_view_finalization/test.py

@@ -0,0 +1,8 @@
+from gs_utils.internal.utils import run_test_driver
+
+
+@run_test_driver
+def driver():
+    # The goal of this test is to exit immediately; this should show a
+    # memory corruption under valgrind if failing.
+    pass

testsuite/tests/V503-022.buffer_view_finalization/test.yaml

@@ -0,0 +1 @@
+title: 'V503-022.buffer_view_finalization'

views/src/buffer_views.adb

@@ -650,7 +650,9 @@ package body Buffer_Views is
       pragma Unreferenced (Column);
 
    begin
-      if Get_MDI (V.Kernel) = null then
+      if V.Kernel.Is_In_Destruction
+        or else Get_MDI (V.Kernel) = null
+      then
          return;
       end if;