Fix macOS crash on editor closed and reopened

On Minihost Modular, the window will not be recreated correctly after
closing. It's because the host does not send the effEditClose opcode.

When effEditOpen is reentered, fVstUI!=nullptr would short-circuit the
initialization of the editor, and continue without an existing window.

distrho/src/DistrhoPluginVST.cpp

@@ -588,7 +588,8 @@ public:
             return 1;
 
         case effEditOpen:
-            if (fVstUI == nullptr)
+            delete fVstUI; // hosts which don't pair effEditOpen/effEditClose calls (Minihost Modular)
+            fVstUI = nullptr;
             {
 # if DISTRHO_OS_MAC
                 if (! fUsingNsView)