Library Manager sometimes throws a NullPointerException

[mistodev]

Description
When opening the library manager and choosing the library type from the drop-down whilst the UI was fetching available libraries, a NullPointerException was thrown.

Would appear to be a race condition as it cannot be replicated reliably.

Stack Trace

java.lang.NullPointerException java.lang.RuntimeException: java.lang.NullPointerException at cc.arduino.contributions.libraries.ui.LibraryManagerUI.lambda$onUpdatePressed$0(LibraryManagerUI.java:212) at java.lang.Thread.run(Thread.java:745) Caused by: java.lang.NullPointerException at com.apple.laf.AquaListUI.repaintCell(AquaListUI.java:154) at com.apple.laf.AquaComboBoxUI$1$1.paintSelected(AquaComboBoxUI.java:117) at com.apple.laf.AquaUtils.blinkMenu(AquaUtils.java:243) at com.apple.laf.AquaComboBoxUI$1.itemStateChanged(AquaComboBoxUI.java:115) at javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1223) at javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1280) at javax.swing.JComboBox.contentsChanged(JComboBox.java:1330) at javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118) at javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93) at javax.swing.DefaultComboBoxModel.addElement(DefaultComboBoxModel.java:131) at javax.swing.JComboBox.addItem(JComboBox.java:716) at cc.arduino.contributions.libraries.ui.LibraryManagerUI.updateUI(LibraryManagerUI.java:166) at processing.app.Base$8.onIndexesUpdated(Base.java:1332) at cc.arduino.contributions.libraries.ui.LibraryManagerUI.lambda$onUpdatePressed$0(LibraryManagerUI.java:210) ... 1 more