* [Context] check Visual before using it

uglcContextGtk2GLX.pas

@@ -351,6 +351,9 @@ var
   attrList: TGLIntArray;
   drawable: PGdkDrawable;
 begin
+  if not Assigned(aControl) then
+    raise EArgumentException.Create('aControl is not assigned');
+
   {
     Temporary (realized) widget to get to display
   }
@@ -382,6 +385,8 @@ begin
       FVisual  := glXChooseVisual(FDisplay, DefaultScreen(FDisplay), @attrList[0]);
     end;
   end;
+  if not Assigned(FVisual) then
+    raise EGLXError.Create('choose visual failed');
 
   {
     Most widgets inherit the drawable of their parent. In contrast to Windows, descending from