Fix jar build

Author: lriggs

vector/src/main/codegen/templates/UnionVector.java

@@ -280,7 +280,10 @@ public StructVector getStruct() {
   <#if minor.class?starts_with("Decimal") || is_timestamp_tz(minor.class) || minor.class == "Duration" || minor.class == "FixedSizeBinary">
   public ${name}Vector get${name}Vector() {
     if (${uncappedName}Vector == null) {
-      throw new IllegalArgumentException("No ${name} present. Provide ArrowType argument to create a new vector");
+      ${uncappedName}Vector = internalStruct.getChild(fieldName(MinorType.${name?upper_case}), ${name}Vector.class);
+      if (${uncappedName}Vector == null) {
+        throw new IllegalArgumentException("No ${name} present. Provide ArrowType argument to create a new vector");
+      }
     }
     return ${uncappedName}Vector;
   }
@@ -307,9 +310,6 @@ public StructVector getStruct() {
 
   public ${name}Vector get${name}Vector(String name) {
     if (${uncappedName}Vector == null) {
-      ${uncappedName}Vector = internalStruct.getChild(fieldName(MinorType.${name?upper_case}), ${name}Vector.class);
-      if (${uncappedName}Vector == null) {
-        throw new IllegalArgumentException("No ${uncappedName} present. Provide ArrowType argument to create a new vector");
       int vectorCount = internalStruct.size();
       ${uncappedName}Vector = addOrGet(name, MinorType.${name?upper_case}, ${name}Vector.class);
       if (internalStruct.size() > vectorCount) {