Fixes 4529: Updated IPart to match new ICapabilityProvider (#4534)

src/api/java/appeng/api/parts/IPart.java

@@ -50,6 +50,7 @@ import net.minecraftforge.api.distmarker.OnlyIn;
 import net.minecraftforge.client.model.data.EmptyModelData;
 import net.minecraftforge.client.model.data.IModelData;
 import net.minecraftforge.common.capabilities.Capability;
+import net.minecraftforge.common.capabilities.CapabilityProvider;
 import net.minecraftforge.common.util.LazyOptional;
 
 import appeng.api.networking.IGridNode;
@@ -346,10 +347,12 @@ public interface IPart extends ICustomCableConnection {
      * capabilities on the cable bus will be forwarded to parts on the appropriate
      * side.
      *
-     * @see TileEntity#getCapability(Capability, net.minecraft.util.Direction)
+     * @see CapabilityProvider#getCapability(Capability,
+     *      net.minecraft.util.Direction)
      *
-     * @return The capability or null.
+     * @return The capability
      */
+    @Nonnull
     default <T> LazyOptional<T> getCapability(Capability<T> capabilityClass) {
         return LazyOptional.empty();
     }

src/main/java/appeng/fluids/helper/DualityFluidInterface.java

@@ -210,7 +210,7 @@ public class DualityFluidInterface
         } else if (capabilityClass == Capabilities.STORAGE_MONITORABLE_ACCESSOR) {
             return (LazyOptional<T>) LazyOptional.of(() -> this.accessor);
         }
-        return null;
+        return LazyOptional.empty();
     }
 
     private boolean hasConfig() {

src/main/java/appeng/tile/networking/CableBusTileEntity.java

@@ -311,7 +311,7 @@ public class CableBusTileEntity extends AEBaseTileEntity implements AEMultiTile
         IPart part = this.getPart(partLocation);
         LazyOptional<T> result = part == null ? LazyOptional.empty() : part.getCapability(capabilityClass);
 
-        if (result != null) {
+        if (result.isPresent()) {
             return result;
         }