Bugfix for loading xml #513

[TatianaBurek]

Pull Request Testing

• Describe testing already performed for these changes:
  
  tested on dakota with this XML
  plot_rhist_DPT2m_nostoch_stoch.xml.txt
  XML should be loaded and the plot should be created

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]

• Do these changes include sufficient testing updates? [Yes or No]

• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by [Fill in date].
  

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