Why am I not able to use the installed help documentation properly after update in MATLAB 9.6 (R2019a)?

Whenever searching any keywords in Help Documentation, there is only empty page by just showing loading image permanently with the following error message in the MATLAB command window.

Exception in thread "com.mathworks.mlwidgets.help.messages.HelpSystemMessageChannel Message Channel Thread": java.lang.NullPointerException

    at com.mathworks.search.lucene.LuceneIndexUtils.indexContainsField(LuceneIndexUtils.java:75)

    at com.mathworks.helpsearch.DocSearchEngineFactory.createAutoSearchEngine(DocSearchEngineFactory.java:50)

    at com.mathworks.help.helpui.search.DocSearchHandler.doSearch(DocSearchHandler.java:59)

    at com.mathworks.help.helpui.search.DocSearchHandler.doSearch(DocSearchHandler.java:52)

    at com.mathworks.mlwidgets.help.messages.search.DocSearchMessageHandler.handleSearch(DocSearchMessageHandler.java:47)

    at com.mathworks.mlwidgets.help.messages.search.DocSearchMessageHandler.handle(DocSearchMessageHandler.java:41)

    at com.mathworks.mlwidgets.html.messages.BrowserMessageSubscriber$HelpMessageRunnable.run(BrowserMessageSubscriber.java:110)

    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)

    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

    at java.lang.Thread.run(Thread.java:748)

The above same issue is also occurred when typing "docsearch plot" at the MATLAB prompt.

Answer 1

[MathWorks Support Team]

There is a bug in MATLAB 9.6 (R2019a) in using both the following 2 settings after installed any update in R2019a which was installed with "English" for "Windows display language".

1. Set "Language" as Languages with a non-latin character set like "Korean" in Preferences > Help

2. Set "Documentation Location" as "Installed locally" in Preferences > Help

To work around this issue, change the “Documentation Location” to “mathworks.com” from “Installed locally” in Preference > Help in MATLAB.

Or, go to http://kr.mathworks.com/help website from your browser and then change the country selection to like “한국 (한국어)” at the bottom.

Answer 2

[Patrik Forssén]

This error is encountered if you add paths to MATLAB that contains a “html” folder, e.g., a third-party toolbox. MATLAB then seems to assume that the “html” folder is help and tries to build a search index in a folder “helpsearch-v3” (R2020a). However, the built index might be corrupt and will then break the help search function and make it throw this Java error. MATLAB should of course check the built index, or at least handle the error in the help search system, but it doesn’t.

The fix is to just remove the “html” and “helpsearch” folders from the MATLAB path.

MATLAB’s help system seems to be very unstable. If it stops working another fix is to exit MATLAB and delete everything in the folder that is returned if you type “prefdir” at the MATLAB command prompt. On Windows typically something like,

C:\Users\<UserName>\AppData\Roaming\MathWorks\MATLAB\<Release>

Note that this will also delete any set preferences.

[Patrik Forssén]

The above applies to all folders that are in the MATLAB path, not only the ones in the installation folder. Type "path" and make a note of all folders that are not in the MATLAB installation folder (i.e., the folder you get from the command "matlabroot"). The open "Set Path" and delete all these paths, save the path and restart MATLAB.

If that helps, then there is a problem in one of the paths that you deleted. Add the deleted paths to the MATLAB path again one by one, restart MATLAB after each addition and see if the problem reappears. If it does the last added folder has a problem and there should be "html" and "helpsearch" folders there. Delete them (or move them to a location not in the MATLAB path) and restart MATLAB.

If it does not help make sure that no startup-scripts has added extra folders to the MATLAB path, i.e., paths outside the installation folder. If that is not the case the MATLAB installation itself is probably corrupt and you have to reinstall MATLAB, delete all content in the folder returned by "prefdir" before you do. First just install MATLAB and check that help search works, then install any toolboxes one by one and check that help search works. If the problem reappears when you do this, you should contact MathWorks as there is problem with help to begin with in the last installed product.