RS if src/href is null in a QTI 2.1 assessment item

2017-04-02 15:25:15,329 [ajp-nio2-8008-exec-10] ERROR Window - OLAT::ERROR ^%^ N8-E12 ^%^ org.olat.core.gui.components 
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Firefox/52.0

**RedScreen** exception while rendering component 'qtirun_cmp' (org.olat.ims.qti21.ui.components.AssessmentTestComponent) listener:[]<br />Message of exception: MethodInvocationException occured while merging template: methName:convertLinkFull, refName:r ::_:: ::_:: ^%^ 
>>> stack of 1.cause::org.olat.core.logging.OLATRuntimeException: exception while rendering component 'qtirun_cmp' (org.olat.ims.qti21.ui.components.AssessmentTestComponent) listener:[]<br />Message of exception: MethodInvocationException occured while merging template: methName:convertLinkFull, refName:r
-> at org.olat.core.gui.components.Window.handleDirties(Window.java:1007)
   at org.olat.core.gui.components.Window.dispatchRequest(Window.java:468)
   at org.olat.core.gui.components.Window.doDispatchRequest(Window.java:242)
   at org.olat.core.gui.components.AbstractComponent.dispatchRequest(AbstractComponent.java:193)
   at org.olat.dispatcher.AuthenticatedDispatcher.processValidDispatchURI(AuthenticatedDispatcher.java:273)
   at org.olat.dispatcher.AuthenticatedDispatcher.execute(AuthenticatedDispatcher.java:190)
   at org.olat.core.servlets.OpenOLATServlet.executeUserRequest(OpenOLATServlet.java:292)
   at org.olat.core.servlets.OpenOLATServlet.doPost(OpenOLATServlet.java:235)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
   at org.olat.core.servlets.OpenOLATServlet.service(OpenOLATServlet.java:178)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
>>> stack of 2.cause::org.olat.core.logging.OLATRuntimeException: MethodInvocationException occured while merging template: methName:convertLinkFull, refName:r
-> at org.olat.core.gui.render.velocity.VelocityHelper.merge(VelocityHelper.java:166)
   at org.olat.core.gui.render.velocity.VelocityHelper.mergeContent(VelocityHelper.java:181)
   at org.olat.ims.qti21.ui.components.AssessmentObjectComponentRenderer.renderVelocity(AssessmentObjectComponentRenderer.java:995)
   at org.olat.ims.qti21.ui.components.AssessmentObjectComponentRenderer.renderInteraction(AssessmentObjectComponentRenderer.java:976)
   at org.olat.ims.qti21.ui.components.AssessmentObjectComponentRenderer.renderBlock(AssessmentObjectComponentRenderer.java:431)
   at org.olat.ims.qti21.ui.components.AssessmentTestComponentRenderer.lambda$renderTestItemBody$1(AssessmentTestComponentRenderer.java:351)
   at java.util.ArrayList.forEach(ArrayList.java:1249)
   at org.olat.ims.qti21.ui.components.AssessmentTestComponentRenderer.renderTestItemBody(AssessmentTestComponentRenderer.java:350)
   at org.olat.ims.qti21.ui.components.AssessmentTestComponentRenderer.renderTestItem(AssessmentTestComponentRenderer.java:224)
   at org.olat.ims.qti21.ui.components.AssessmentTestComponentRenderer.renderTestEvent(AssessmentTestComponentRenderer.java:164)
   at org.olat.ims.qti21.ui.components.AssessmentTestComponentRenderer.render(AssessmentTestComponentRenderer.java:126)
>>> stack of 3.cause::java.lang.NullPointerException: null
-> at org.olat.ims.qti21.ui.components.AssessmentRenderFunctions.convertLink(AssessmentRenderFunctions.java:622)
   at org.olat.ims.qti21.ui.components.AssessmentObjectVelocityRenderDecorator.convertLinkFull(AssessmentObjectVelocityRenderDecorator.java:171)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.lang.reflect.Method.invoke(Method.java:498)
   at org.apache.velocity.util.introspection.UberspectImpl$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)
   at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:280)
   at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:369)