[NicolaIsotta]

Sometimes, while renaming java variables, an exception is thrown and a popup is shown. The exception is not harmful (renaming happens anyway), but it's quite annoying. This PR avoids the NPE with a null check (just as line 164).

Exception stack trace

SEVERE [org.openide.util.Exceptions]
java.lang.NullPointerException: Cannot invoke "javax.lang.model.element.VariableElement.getEnclosingElement()" because "var" is null
	at org.netbeans.modules.j2ee.jpa.refactoring.rename.RelationshipMappingRename$1.run(RelationshipMappingRename.java:126)
	at org.netbeans.modules.j2ee.jpa.refactoring.rename.RelationshipMappingRename$1.run(RelationshipMappingRename.java:117)
	at org.netbeans.api.java.source.JavaSource$MultiTask.run(JavaSource.java:500)
	at org.netbeans.modules.parsing.impl.TaskProcessor.callUserTask(TaskProcessor.java:586)
	at org.netbeans.modules.parsing.api.ParserManager$UserTaskAction.run(ParserManager.java:197)
	at org.netbeans.modules.parsing.api.ParserManager$UserTaskAction.run(ParserManager.java:180)
	at org.netbeans.modules.parsing.impl.TaskProcessor$2.call(TaskProcessor.java:181)
	at org.netbeans.modules.parsing.impl.TaskProcessor$2.call(TaskProcessor.java:178)
	at org.netbeans.modules.masterfs.filebasedfs.utils.FileChangedManager.priorityIO(FileChangedManager.java:153)
	at org.netbeans.modules.masterfs.providers.ProvidedExtensions.priorityIO(ProvidedExtensions.java:341)
	at org.netbeans.modules.parsing.nb.DataObjectEnvFactory.runPriorityIO(DataObjectEnvFactory.java:118)
	at org.netbeans.modules.parsing.impl.Utilities.runPriorityIO(Utilities.java:67)
	at org.netbeans.modules.parsing.impl.TaskProcessor.runUserTask(TaskProcessor.java:178)
	at org.netbeans.modules.parsing.api.ParserManager.parse(ParserManager.java:83)
	at org.netbeans.api.java.source.JavaSource.runUserActionTaskImpl(JavaSource.java:450)
	at org.netbeans.api.java.source.JavaSource.runUserActionTask(JavaSource.java:421)
	at org.netbeans.modules.j2ee.jpa.refactoring.rename.RelationshipMappingRename.prepare(RelationshipMappingRename.java:117)
[catch] at org.netbeans.modules.refactoring.api.AbstractRefactoring.pluginsPrepare2(AbstractRefactoring.java:417)
	at org.netbeans.modules.refactoring.api.AbstractRefactoring.pluginsPrepare(AbstractRefactoring.java:401)
	at org.netbeans.modules.refactoring.api.AbstractRefactoring.prepare(AbstractRefactoring.java:212)
	at org.netbeans.modules.refactoring.spi.impl.ParametersPanel$Prepare.run(ParametersPanel.java:1063)
	at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1370)
	at org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:45)
	at org.openide.util.lookup.Lookups.executeWith(Lookups.java:287)
	at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:1981)

note 1: added Java label, as the j2ee.jpa.refactoring module is in the java cluster, but it seems the module is not tested anywhere

note 2: from a quick eyeballing, the class does actually nothing at all: the Set<ElementHandle<TypeElement>> set is always empty. Plus, it only handles the old javax package. I don't use JPA so I don't intend to work on it, but it was worth noticing

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