- make document charges and allowances serializable

library/src/main/java/org/mustangproject/Allowance.java

@@ -1,5 +1,7 @@
 package org.mustangproject;
 
+import com.fasterxml.jackson.annotation.JsonIgnore;
+
 import java.math.BigDecimal;
 
 /***
@@ -28,6 +30,7 @@ public class Allowance extends Charge {
 	 * @return false since its not supposed to be calculated negatively
 	 */
 	@Override
+	@JsonIgnore
 	public boolean isCharge() {
 		return false;
 	}

library/src/main/java/org/mustangproject/Charge.java

@@ -1,5 +1,6 @@
 package org.mustangproject;
 
+import com.fasterxml.jackson.annotation.JsonIgnore;
 import org.mustangproject.ZUGFeRD.IAbsoluteValueProvider;
 import org.mustangproject.ZUGFeRD.IZUGFeRDAllowanceCharge;
 
@@ -151,6 +152,7 @@ public class Charge implements IZUGFeRDAllowanceCharge {
 	 * @return true since it is supposed to be calculated negatively
 	 */
 	@Override
+	@JsonIgnore
 	public boolean isCharge() {
 		return true;
 	}

library/src/main/java/org/mustangproject/Invoice.java

@@ -522,6 +522,20 @@ public class Invoice implements IExportableTransaction {
 		}
 	}
 
+	/***
+	 * this is wrong and only used from jackson
+	 * @param iza
+	 * @return
+	 */
+	public Invoice setZFAllowances(Allowance[] iza) {
+		Allowances=new ArrayList<>();
+
+		for (IZUGFeRDAllowanceCharge cz:iza) {
+			Allowances.add(cz);
+		}
+		return this;
+	}
+
 
 	@Override
 	public IZUGFeRDAllowanceCharge[] getZFCharges() {
@@ -532,6 +546,18 @@ public class Invoice implements IExportableTransaction {
 		}
 	}
 
+	/***
+	 * this is wrong and only used from jackson
+	 * @param iza
+	 * @return
+	 */
+	public Invoice setZFCharges(Charge[] iza) {
+		Charges=new ArrayList<>();
+		for (IZUGFeRDAllowanceCharge cz:iza) {
+			Charges.add(cz);
+		}
+		return this;
+	}
 
 	@Override
 	public IZUGFeRDAllowanceCharge[] getZFLogisticsServiceCharges() {

library/src/main/java/org/mustangproject/ZUGFeRD/LineCalculator.java

@@ -34,9 +34,13 @@ public class LineCalculator {
 			}
 		}
 		
-		BigDecimal vatPercent = currentItem.getProduct().getVATPercent();
-		if (vatPercent == null)
+		BigDecimal vatPercent = null;
+		if (currentItem.getProduct()!=null) {
+			vatPercent = currentItem.getProduct().getVATPercent();
+		}
+		if (vatPercent == null) {
 			vatPercent = BigDecimal.ZERO;
+		}
 		BigDecimal multiplicator = vatPercent.divide(BigDecimal.valueOf(100));
 		priceGross = currentItem.getPrice(); // see https://github.com/ZUGFeRD/mustangproject/issues/159
 		price = priceGross.subtract(allowance).add(charge);