Fix isamill NPE & chemplant structure check (#857)

fix

Author: HoleFish

src/main/java/gtPlusPlus/xmod/gregtech/common/tileentities/machines/multi/processing/GregtechMetaTileEntity_IsaMill.java

@@ -431,7 +431,10 @@ public int getMaxParallelRecipes() {
     public ArrayList<ItemStack> getStoredInputs() {
         ArrayList<ItemStack> tItems = super.getStoredInputs();
         for (GT_MetaTileEntity_Hatch_MillingBalls tHatch : filterValidMTEs(mMillingBallBuses)) {
-            tItems.addAll(tHatch.getContentUsageSlots());
+            AutoMap<ItemStack> aHatchContent = tHatch.getContentUsageSlots();
+            if (!aHatchContent.isEmpty()) {
+                tItems.addAll(aHatchContent);
+            }
         }
         return tItems;
     }

src/main/java/gtPlusPlus/xmod/gregtech/common/tileentities/machines/multi/production/chemplant/GregtechMTE_ChemicalPlant.java

@@ -339,6 +339,7 @@ public boolean checkMachine(IGregTechTileEntity aBaseMetaTileEntity, ItemStack a
         mMachineCasingTier = 0;
         mPipeCasingTier = 0;
         mCoilTier = 0;
+        maxTierOfHatch = 0;
         mCatalystBuses.clear();
         setCoilMeta(HeatingCoilLevel.None);
         if (checkPiece(mName, 3, 6, 0) && mCasing >= 70) {