`pop_importbids` fails silently or crashes when input `bidsfolder` is a MATLAB `string` object (requires `char`)

Description:

The function pop_importbids fails to correctly locate or parse datasets when the input path bidsfolder is provided as a MATLAB string (double quotes "..."), which is the default text type in modern MATLAB versions. It only functions correctly when the input is a char array (single quotes '...').

The current documentation header describes the input simply as [string], which is ambiguous in the context of modern MATLAB (where it usually implies the string class, but legacy code expects char).

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Steps to Reproduce:

Provide a valid BIDS folder path as a double-quoted string.

Run pop_importbids.

% Assume valid BIDS folder at this path, we can clone one from OpenNeuro
bidsPath = "C:\data\my_bids_dataset"; % MATLAB string object

% This fails (crashes or returns 'No dataset found')
[STUDY, ALLEEG] = pop_importbids(bidsPath);

% This works
[STUDY, ALLEEG] = pop_importbids(char(bidsPath));

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Root Cause Analysis:

The crash occurs due to legacy indexing logic that assumes character array behavior. Specifically, around line 605 inside the file loop:

% Existing code assumes eegFileRaw is char
infoData = bids_importjson([ eegFileRaw(1:end-8) '_' modality '.json' ], ...);

• If char: eegFileRaw(1:end-8) correctly slices the filename string (removing the last 8 characters).

• If string: eegFileRaw is a scalar string object (1x1). end evaluates to 1. The range 1:end-8 becomes 1:-7 (empty). This results in an empty array being passed to bids_importjson, causing downstream errors (e.g., "Index exceeds array elements" or file not found).

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Suggested Fix:

1. Documentation Update (Low Effort) Explicitly specify in the help section that inputs must be character arrays.

% Inputs:
%   bidsfolder - [char array] Path to the BIDS folder (e.g., 'C:\data\exp'). 
%                Do not use MATLAB string objects ("...").

2. Robust Input Handling (Recommended) Add a simple cast at the beginning of pop_importbids to ensure compatibility with modern MATLAB scripts without requiring user intervention:

if nargin > 0
    bidsfolder = char(bidsfolder);
end

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Additional Context on Type Dependency:

While this report focuses on pop_importbids, it is worth noting that string vs char incompatibilities appear to be systemic within the ecosystem invoked here. For example, if we write eegFileRaw = char(eegFileRaw) at line 604 (to fix the line 605 crash), downstream functions like pop_saveset may subsequently fail due to strict type checks (e.g., strcmpi failures on string properties). Explicit input sanitization (casting to char) at the entry point of pop_importbids would effectively shield the internal logic from these external type mismatches.