Fix NPE in SpringReference

[Laurens-W]

What's your motivation?

Failure reported on platform

Anyone you would like to review specifically?

@knutwannheden

Have you considered any alternatives or workarounds?

Any additional context

Error:

Message:

Cannot invoke "org.openrewrite.xml.tree.Xml$Tag.getAttributes()" because the return value of "org.openrewrite.xml.tree.Xml$Document.getRoot()" is null

Detail:

java.lang.NullPointerException: Cannot invoke "org.openrewrite.xml.tree.Xml$Tag.getAttributes()" because the return value of "org.openrewrite.xml.tree.Xml$Document.getRoot()" is null
org.openrewrite.xml.trait.SpringReference$Provider.isAcceptable(SpringReference.java:118)
org.openrewrite.SourceFileWithReferences$References.lambda$build$0(SourceFileWithReferences.java:59)
java.base/java.lang.Iterable.forEach(Iterable.java:75)
org.openrewrite.SourceFileWithReferences$References.build(SourceFileWithReferences.java:58)
org.openrewrite.xml.tree.Xml$Document.getReferences(Xml.java:179)
org.openrewrite.java.search.UsesType.visit(UsesType.java:124)
org.openrewrite.TreeVisitor.visitNonNull(TreeVisitor.java:170)
org.openrewrite.java.ChangeType$1.preVisit(ChangeType.java:95)
org.openrewrite.java.ChangeType$1.preVisit(ChangeType.java:83)
org.openrewrite.TreeVisitor.visit(TreeVisitor.java:247)
org.openrewrite.TreeVisitor.visit(TreeVisitor.java:157)
org.openrewrite.Preconditions$Check.visit(Preconditions.java:174)
org.openrewrite.Preconditions$Check.visit(Preconditions.java:145)
org.openrewrite.scheduling.RecipeRunCycle.lambda$editSources$6(RecipeRunCycle.java:186)
io.micrometer.core.instrument.AbstractTimer.recordCallable(AbstractTimer.java:178)
org.openrewrite.table.RecipeRunStats.recordEdit(RecipeRunStats.java:67)
...

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files