Core - Corrected disposable pattern usage

Core - Further nullable refactoring..

src/Artemis.Core/Plugins/LayerBrushes/Internal/BaseLayerBrush.cs

@@ -97,19 +97,32 @@ namespace Artemis.Core.LayerBrushes
 
         internal abstract void InternalRender(SKCanvas canvas, SKRect path, SKPaint paint);
 
-        internal virtual void Dispose(bool disposing)
+        #region IDisposable
+
+        /// <summary>
+        ///     Releases the unmanaged resources used by the object and optionally releases the managed resources.
+        /// </summary>
+        /// <param name="disposing">
+        ///     <see langword="true" /> to release both managed and unmanaged resources;
+        ///     <see langword="false" /> to release only unmanaged resources.
+        /// </param>
+        protected virtual void Dispose(bool disposing)
         {
+            if (disposing)
+            {
+                DisableLayerBrush();
+                BaseProperties?.Dispose();
+            }
         }
 
         /// <inheritdoc />
         public void Dispose()
         {
-            DisableLayerBrush();
-            BaseProperties?.Dispose();
-
             Dispose(true);
             GC.SuppressFinalize(this);
         }
+
+        #endregion
     }
 
     /// <summary>

src/Artemis.Core/Plugins/LayerBrushes/Internal/PropertiesLayerBrush.cs

@@ -26,7 +26,7 @@ namespace Artemis.Core.LayerBrushes
             {
                 // I imagine a null reference here can be confusing, so lets throw an exception explaining what to do
                 if (_properties == null)
-                    throw new ArtemisPluginException("Cannot access brush properties until OnPropertiesInitialized has been called");
+                    throw new InvalidOperationException("Cannot access brush properties until OnPropertiesInitialized has been called");
                 return _properties;
             }
             internal set => _properties = value;