I try to load a simple groovy script inside my pipeline definition.
The following is my Jenkins-Pipeline-Script:

def flow

stage(name: "Fetch groovy script from SCM")
{
node("master")
{
ws('C:\\Jenkins\\jobs\\BuildJobs\\jobs\\Build_Test
workspace')
{ flow = load 'build.groovy' println(flow) }
}
}

flow.execute()

It loads the script C:\Jenkins\jobs\BuildJobs\jobs\Build_Test\workspace\build.groovy on the master node:

def execute()
{
stage (name:"Print ReleaseTypes")
{
node("master")
{ echo releaseType }
}
}

return this

If I execute the pipeline job, I get the following result:

[Pipeline] stage
[Pipeline] { (Fetch groovy script from SCM)
[Pipeline] node
Running on master in C:\jenkins\jobs\BuildJobs\jobs\Build_Test\workspace
[Pipeline] {
[Pipeline] ws
Running in C:\Jenkins\jobs\BuildJobs\jobs\Build_Test\workspace
[Pipeline] {
[Pipeline] load
[Pipeline] { (build.groovy) [Pipeline] }
[Pipeline] // load
[Pipeline] echo
null
[Pipeline] }
[Pipeline] // ws
[Pipeline] }
[Pipeline] // node
[Pipeline] }
[Pipeline] // stage
[Pipeline] End of Pipeline
[BFA] Scanning build for known causes...
[BFA] No failure causes found
[BFA] Done. 0s
java.lang.NullPointerException: Cannot invoke method execute() on null object

I use the lates Jenkins (Jenkins ver. 2.26)