SAFE_POINT cancel the tree building subtask

1. Open _common_data/clustal/10000_sequences.aln
   Expected: the Alignment Editor appears.
2. Open the Task View.
3. Build tree:
   1. click Build Tree,
      expected: the "Build Phylogenetic Tree" dialog appears;
   2. select the FastTree tree building method;
   3. click Build.
      Expected: the "Calculating Phylogenetic Tree" task has started.
4. Cancel the subtask:
   1. in the Task View, expand the main tree building task;
   2. select the "Run FastTree tool" subtask;
   3. right-click it, and click the Cancel Task option.
      Expected: the main tree building task is successfully canceled without redundant messages.
      Current state: in the Log:

      [ERROR][15:51] Trying to recover from error: Tree is not present! at C:\BuildAgent\work\release\ugene\src\corelibs\U2Algorithm\src\phyltree\PhyTreeGeneratorTask.cpp:73
      [ERROR][15:51] NULL tree data
      

The same with other subtasks or the IQ-TREE tool.