GH-326 - prevent NPE when treeTable has not been initialized yet

Author: jisedlac

visualvm/heapviewer/src/org/graalvm/visualvm/heapviewer/ui/TreeTableView.java

@@ -278,13 +278,15 @@ public void setColumnVisible(DataType dataType, boolean visible) {
         int column = getColumn(dataType);
         if (column == -1) return;
 
+        if (treeTable == null) return;
         treeTable.setColumnVisibility(column, visible);
     }
 
     public boolean isColumnVisible(DataType dataType) {
         int column = getColumn(dataType);
         if (column == -1) return false;
 
+        if (treeTable == null) return true; // assume initially visible
         return treeTable.isColumnVisible(column);
     }