docking, initialize a `var` parameter used as result

it's not `out` so there is no guarantee that the caller
has initialized it.

src/u_main.pas

@@ -1620,6 +1620,7 @@ procedure TMainForm.anchorDockingAddControlEvent(Sender: TObject; aName: string;
             var AControl: TControl; DoDisableAutoSizing: boolean);
 var w : TDexedWidget;
 begin
+  AControl := nil;
   for w in fWidgList do
     if w.name = aName then
     begin