This is a known bug. It comes and goes. One of your open files has not been saved correctly. The editor thinks it is saved, but the IDE somehow knows the environment is dirty.
Workaround/Remedy
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One at a time, close your files.
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When you close a single file, open it again.
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Look for an asterisk in the tab
3.1 If there is no asterisk, close the file and move on to the next file. Repeat from Step 1 above.
3.2 If there is an asterisk in the tab, check your changes are present then save the file again:
Now the asterisk should be gone: 
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Repeat from Step 1 for each open file.
@ThanhTo @duyluong guys, this bug has cobwebs on it. Please fix!