I updated "pipeline model api" pluggin from 1.1.9 to 1.2 and my jobs stopped working. I am getting the following error:
java.lang.AbstractMethodError: org.jenkinsci.plugins.pipeline.modeldefinition.validator.ModelValidatorImpl.validateElement(Lorg/jenkinsci/plugins/pipeline/modeldefinition/ast/ModelASTStage;Z)Z
at org.jenkinsci.plugins.pipeline.modeldefinition.ast.ModelASTStage.validate(ModelASTStage.java:78)
at org.jenkinsci.plugins.pipeline.modeldefinition.ast.ModelASTStages.validate(ModelASTStages.java:41)
at org.jenkinsci.plugins.pipeline.modeldefinition.ast.ModelASTStages.validate(ModelASTStages.java:35)
at org.jenkinsci.plugins.pipeline.modeldefinition.ast.ModelASTPipelineDef.validate(ModelASTPipelineDef.java:64)
at org.jenkinsci.plugins.pipeline.modeldefinition.ast.ModelASTPipelineDef$validate.call(Unknown Source)
at org.jenkinsci.plugins.pipeline.modeldefinition.parser.ModelParser.parse(ModelParser.groovy:204)
at org.jenkinsci.plugins.pipeline.modeldefinition.parser.ModelParser$parse.callCurrent(Unknown Source)
at org.jenkinsci.plugins.pipeline.modeldefinition.parser.ModelParser.parse(ModelParser.groovy:91)
at org.jenkinsci.plugins.pipeline.modeldefinition.parser.GroovyShellDecoratorImpl$1.call(GroovyShellDecoratorImpl.java:46)
at org.codehaus.groovy.control.CompilationUnit.applyToPrimaryClassNodes(CompilationUnit.java:1065)
at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:603)
at org.codehaus.groovy.control.CompilationUnit.processPhaseOperations(CompilationUnit.java:581)
at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:558)
at groovy.lang.GroovyClassLoader.doParseClass(GroovyClassLoader.java:298)
at groovy.lang.GroovyClassLoader.parseClass(GroovyClassLoader.java:268)
at groovy.lang.GroovyShell.parseClass(GroovyShell.java:688)
at groovy.lang.GroovyShell.parse(GroovyShell.java:700)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.doParse(CpsGroovyShell.java:129)
at org.jenkinsci.plugins.workflow.cps.CpsGroovyShell.reparse(CpsGroovyShell.java:123)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.parseScript(CpsFlowExecution.java:517)
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.start(CpsFlowExecution.java:480)
at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:269)
at hudson.model.ResourceController.execute(ResourceController.java:97)
at hudson.model.Executor.run(Executor.java:419)
Finished: FAILURE
I reverted the plugin version to 1.1.9 and everything is functional again. I know close to nothing about java and not much more about Jenkins so I can't really help debugging that

If you want me to do any other test, I will be glad to do that.

Side question: Would it be possible that a dependency was ommited and in facts I need to upgrade Pipeline: Declarative Extension Points API and/or Pipeline: Model Definition from 1.1.9 to 1.2 at the same time?

I am reluctant to do that as there is a warning stating I will not be able to go back if it does not work...