Error while computing the migration plan

...
Caused by: org.xwiki.velocity.XWikiVelocityException: Failed to evaluate content with id [xwiki:NestedPagesMigration.Service]
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:224)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:198)
	at org.xwiki.rendering.internal.macro.velocity.VelocityMacro.evaluateString(VelocityMacro.java:124)
	... 183 more
Caused by: org.apache.velocity.exception.MethodInvocationException: Invocation of method 'getSerializedPlan' in  class org.xwiki.contrib.nestedpagesmigrator.script.NestedPagesMigratorScriptService threw exception java.lang.NullPointerException at 72:xwiki:NestedPagesMigration.Service[line 87, column 31]
	at org.apache.velocity.runtime.parser.node.ASTMethod.handleInvocationException(ASTMethod.java:243)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:187)
	at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:280)
	at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:369)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:216)
	at org.apache.velocity.runtime.directive.RuntimeMacro.render(RuntimeMacro.java:311)
	at org.apache.velocity.runtime.directive.RuntimeMacro.render(RuntimeMacro.java:230)
	at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:207)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTElseIfStatement.render(ASTElseIfStatement.java:92)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:342)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluateInternal(DefaultVelocityEngine.java:256)
	at org.xwiki.velocity.internal.DefaultVelocityEngine.evaluate(DefaultVelocityEngine.java:222)
	... 185 more
Caused by: java.lang.NullPointerException
	at org.xwiki.contrib.nestedpagesmigrator.MigrationPlanSerializer.serialize(MigrationPlanSerializer.java:83)
	at org.xwiki.contrib.nestedpagesmigrator.script.NestedPagesMigratorScriptService.getSerializedPlan(NestedPagesMigratorScriptService.java:105)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:395)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:384)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:173)
	... 198 more