[NUI.Gadget] Modify NUIGadgetAssembly unload logic

_assembly must be reset to null after calling Unload().

Signed-off-by: Changgyu Choi <[email protected]>

src/Tizen.NUI.Gadget/Tizen.NUI/NUIGadgetAssembly.cs

@@ -104,6 +104,7 @@ internal void Unload()
                     (_assemblyRef.Target as NUIGadgetAssemblyLoadContext).Unload();
                 }
 
+                _assembly = null;
                 _loaded = false;
                 Log.Warn("Unload(): " + _assemblyPath + " --");
             }