Why does the BIML ScriptTask fail until I open the script in VS and save the package

I have this BIML Script, which creates a Master and child package. The child package has a ScriptTask in it that maps the value of a parent package variable to a child package variable. When I Generate the packages in SSIS (Visual Studio 2015, target Sql Server 2014) they generate fine. If I run the master package, the child fails with this error:

DTS Script Task has encountered an exception in user code:
Project name:  ST_GetJobLogIdHere
Exception has been thrown by the target of an invocation.

at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
at Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTATaskScriptingEngine.ExecuteScript()

When I open the ScriptTask in the child, then close it again, the package is dirty so I save it. After that, running the packages works properly.

Here's the BIML Script:

<Biml xmlns="http://schemas.varigence.com/biml.xsd">

    <ScriptProjects>
        <ScriptTaskProject ProjectCoreName="ST_GetJobLogId" Name="ST_GetJobLogIdScript" VstaMajorVersion="0">
            <ReadOnlyVariables>
                <Variable Namespace="User" VariableName="vMasterJobLogId" DataType="Int32" />
            </ReadOnlyVariables>
            <ReadWriteVariables>
                <Variable Namespace="User" VariableName="vJobLogId" DataType="Int32" />
            </ReadWriteVariables>
            <AssemblyReferences>
                <AssemblyReference AssemblyPath="Microsoft.SqlServer.ManagedDTS" />
                <AssemblyReference AssemblyPath="Microsoft.SqlServer.ScriptTask" />
                <AssemblyReference AssemblyPath="System" />
                <AssemblyReference AssemblyPath="System.Data" />
                <AssemblyReference AssemblyPath="System.Windows.Forms" />
                <AssemblyReference AssemblyPath="System.Xml" />
            </AssemblyReferences>
            <Files>
                <File Path="AssemblyInfo.cs" BuildAction="Compile">
                    using System.Reflection;

                    [assembly: AssemblyTitle("ST_GetJobLogId")]
                    [assembly: AssemblyDescription("")]
                    [assembly: AssemblyConfiguration("")]
                    [assembly: AssemblyCompany("")]
                    [assembly: AssemblyProduct("ST_GetJobLogId")]
                    [assembly: AssemblyCopyright("Copyright @ BMI 2017")]
                    [assembly: AssemblyTrademark("")]
                    [assembly: AssemblyCulture("")]
                    [assembly: AssemblyVersion("1.0.*")]
                </File>
                <File Path="ScriptMain.cs" BuildAction="Compile">
                    using Microsoft.SqlServer.Dts.Runtime;

                    namespace ST_GetJobLogId
                    {
                        [Microsoft.SqlServer.Dts.Tasks.ScriptTask.SSISScriptTaskEntryPoint]
                        public partial class ScriptMain : Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTARTScriptObjectModelBase
                        {
                            public void Main()
                            {
                                int jobLogId = (int)Dts.Variables["User::vMasterJobLogId"].Value;
                                if (jobLogId != 0)
                                    Dts.Variables["User::vJobLogId"].Value = jobLogId;
                                Dts.TaskResult = (int)ScriptResults.Success;
                            }
                            enum ScriptResults
                            {
                                Success = DTSExecResult.Success,
                                Failure = DTSExecResult.Failure
                            };
                        }
                    }
                </File>
            </Files>
        </ScriptTaskProject>
    </ScriptProjects>

    <Packages>
        <Package Name="Master" ProtectionLevel="EncryptSensitiveWithUserKey">
            <Variables>
                <Variable Name="vMasterJobLogId" DataType="Int32">1</Variable>
            </Variables>
            <Tasks>
                <ExecutePackage Name="PKG - Execute FixMe">
                    <ExternalProjectPackage Package="FixMe.dtsx" />
                </ExecutePackage>
            </Tasks>
        </Package>
        <Package Name="FixMe" ProtectionLevel="EncryptSensitiveWithUserKey">
            <Variables>
                <Variable Name="vJobLogId" DataType="Int32">0</Variable>
            </Variables>
            <Tasks>
                <Script Name="SCR - Get vJobLogId from parent vMasterJobLogId" ProjectCoreName="ST_GetJobLogIdHere" >
                    <ScriptTaskProjectReference ScriptTaskProjectName="ST_GetJobLogIdScript" />
                </Script>
            </Tasks>
        </Package>
    </Packages>

</Biml>

What am I missing? Why is the package failing until I open the code window manually? Thanks in advance for your help, Simon

Read the comments above for clarification of the question.

It turns out that BIML Express/BidsHelper in Visual Studio 2015 does not know which target version of SQL Server is set in the Project properties.

The solution was to install VS2013 / SSDT 2014, and BIML Express for VS2013 to be able to generate the SSIS 2014 packages properly.

VS 2015 / SSDT / BIML Express will only generate Script Tasks for SQL Server SSIS 2016 packages.

Thanks to Bill for the answer.