attempt to fix #253

src/main/java/appeng/core/api/definitions/ApiBlocks.java

@@ -521,6 +521,10 @@ public final class ApiBlocks implements IBlocks {
                 .disableItem()
                 .build();
 
+        if (!doubleSlabDef.maybeBlock().isPresent()) {
+            return new BlockDefinition(slabId, null, null);
+        }
+
         Verify.verify(doubleSlabDef.maybeBlock().isPresent());
 
         BlockSlab doubleSlabBlock = (BlockSlab) doubleSlabDef.maybeBlock().get();
@@ -530,6 +534,10 @@ public final class ApiBlocks implements IBlocks {
                 .features(AEFeature.DECORATIVE_BLOCKS)
                 .build();
 
+        if (!itemDef.maybeItem().isPresent()) {
+            return new BlockDefinition(slabId, null, null);
+        }
+
         Verify.verify(itemDef.maybeItem().isPresent());
 
         // Return a new composite block definition that combines the single slab block with the slab item