Fixes NPE on empty tooltips (#4546)

src/main/java/appeng/client/gui/widgets/CustomSlotWidget.java

@@ -8,6 +8,7 @@ import net.minecraft.client.gui.AbstractGui;
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.item.ItemStack;
 import net.minecraft.util.text.ITextComponent;
+import net.minecraft.util.text.StringTextComponent;
 
 public abstract class CustomSlotWidget extends AbstractGui implements ITooltip {
     private final int x;
@@ -39,7 +40,7 @@ public abstract class CustomSlotWidget extends AbstractGui implements ITooltip {
 
     @Override
     public ITextComponent getMessage() {
-        return null;
+        return StringTextComponent.EMPTY;
     }
 
     @Override

src/main/java/appeng/client/gui/widgets/ITooltip.java

@@ -18,7 +18,10 @@
 
 package appeng.client.gui.widgets;
 
+import javax.annotation.Nonnull;
+
 import net.minecraft.util.text.ITextComponent;
+import net.minecraft.util.text.StringTextComponent;
 
 /**
  * AEBaseGui controlled Tooltip Interface.
@@ -26,10 +29,13 @@ import net.minecraft.util.text.ITextComponent;
 public interface ITooltip {
 
     /**
-     * returns the tooltip message.
+     * Returns the tooltip message.
+     * 
+     * Should use {@link StringTextComponent#EMPTY} for no tooltip
      *
      * @return tooltip message
      */
+    @Nonnull
     ITextComponent getMessage();
 
     /**