[Avalonia.Native OSX] fix api for clipboard

native/Avalonia.Native/inc/avalonia-native.h

@@ -321,7 +321,7 @@ AVNCOM(IAvnScreens, 0e) : IUnknown
 
 AVNCOM(IAvnClipboard, 0f) : IUnknown
 {
-    virtual IAvnString* GetText () = 0;
+    virtual HRESULT GetText (IAvnString**ppv) = 0;
     virtual HRESULT SetText (void* utf8Text) = 0;
     virtual HRESULT Clear() = 0;
 };

native/Avalonia.Native/src/OSX/clipboard.mm

@@ -8,11 +8,18 @@ class Clipboard : public ComSingleObject<IAvnClipboard, &IID_IAvnClipboard>
 {
 public:
     FORWARD_IUNKNOWN()
-    virtual IAvnString* GetText () override
+    virtual HRESULT GetText (IAvnString**ppv) override
     {
         @autoreleasepool
         {
-            return CreateAvnString([[NSPasteboard generalPasteboard] stringForType:NSPasteboardTypeString]);
+            if(ppv == nullptr)
+            {
+                return E_POINTER;
+            }
+            
+            *ppv = CreateAvnString([[NSPasteboard generalPasteboard] stringForType:NSPasteboardTypeString]);
+            
+            return S_OK;
         }
     }