macosx: fixed race conditional crashes in PXSourceList (close #7171)

Trust the ObjC runtime, it will do its job properly.

modules/gui/macosx/PXSourceList.m

@@ -71,10 +71,6 @@ NSString * const PXSLDeleteKeyPressedOnRowsNotification = @"PXSourceListDeleteKe
 
 - (void)dealloc
 {
-    //Remove ourselves as the delegate and data source to be safe
-    [super setDataSource:nil];
-    [super setDelegate:nil];
-
     //Unregister the delegate from receiving notifications
     [[NSNotificationCenter defaultCenter] removeObserver:_secondaryDelegate name:nil object:self];
 
@@ -83,10 +79,6 @@ NSString * const PXSLDeleteKeyPressedOnRowsNotification = @"PXSourceListDeleteKe
 
 - (void)finalize
 {
-    //Remove ourselves as the delegate and data source to be safe
-    [super setDataSource:nil];
-    [super setDelegate:nil];
-
     //Unregister the delegate from receiving notifications
     [[NSNotificationCenter defaultCenter] removeObserver:_secondaryDelegate name:nil object:self];
 
@@ -117,6 +109,7 @@ NSString * const PXSLDeleteKeyPressedOnRowsNotification = @"PXSourceListDeleteKe
 {
     _secondaryDataSource = aDataSource;
 
+    if ([self respondsToSelector:@selector(reloadData)])
         [self reloadData];
 }
 
@@ -138,6 +131,7 @@ NSString * const PXSLDeleteKeyPressedOnRowsNotification = @"PXSourceListDeleteKe
 
 - (void)reloadData
 {
+    if ([super respondsToSelector:@selector(reloadData)])
         [super reloadData];
 
     //Expand items that are displayed as always expanded