How do I access a MATLAB UI function in another .m file?

I am attempting to declutter my MATLAB app code by separating some of the initialization into separate .m files. For this I have set up various files for each type of component (e.g. a file for buttons, graph, etc.). I am attempting to access a function in my master initialize file from the file for buttons. My code goes as follows in the buttons .m file goes as follow:

classdef buttons < handle
    methods
        %initializes the UI
        function buttonCreate(app)
           
            %Create file load 1
            app.fileload1 = uibutton(app.gridLayout, 'push');
            app.fileload1.FontSize = 36;
            app.fileload1.Layout.Row = [8 9];
            app.fileload1.Layout.Column = 1;
            app.fileload1.Text = 'Load 1';

            %proceeds to create the rest of the buttons
        end
    end
end

Now I attempt to access the buttonCreate() function in my master initialize file initialize.m:

classdef initialize < handle
    properties 
        fig
        gridLayout
        axes

        fileload1
    end
    methods
        %initializes the UI
        function init(app)
            %create canvas
            import buttons.*;
            fig = uifigure;
            fig.Position = [100 100 1920 1080];
            movegui(fig,'center');
            fig.Name = "Audio Editor";

            %Create grid layout
            gridLayout = uigridlayout(fig);
            gridLayout.ColumnWidth = {'1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x'};
            gridLayout.RowHeight = {'1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x', '1x'};

            buttonCreate(app);

        end
    end
    %code for calling and deleting
    methods
        %calls code to create canvas upon app start
        function app = initialize
            init(app)
        end
        %removes the app and deletes app.fig
        function delete(app)
            delete(app.fig);
        end
    end
end

Error in initialize/init (line 41)
            buttonCreate(app);
            ^^^^^^^^^^^^^^^^^^
Error in initialize (line 54)
            init(app)
            ^^^^^^^^^

This resulted in the UIFigure still being created, but with no button and the terminal giving the error given above.

Seems like you're doing some sort of combination of two distinct things:

1. Use a simple function, which can be in its own .m file or one of the methods of your main class:

% in your initialise class constructor you still have
buttonCreate(app);

% In buttonCreate.m
function buttonCreate(app)           
    % Create file load 1
    app.fileload1 = uibutton(app.gridLayout, 'push');
    app.fileload1.FontSize = 36;
    app.fileload1.Layout.Row = [8 9];
    % ...
end

2. Use a class to hold references to all of your buttons, in which case the objects you create would be properties of that class, rather than the main app object:

% in your initialise class constructor you create a buttons
% object and assign it to the buttons property.
% The buttons don't need to know anything about the app, just
% the target grid layout to parent the buttons
app.buttons = buttons( app.gridLayout );

% In buttons.m
classdef buttons < handle
    properties
      fileload1
    end
    methods
        function obj = buttons( grid )           
            % Create file load 1
            obj.fileload1 = uibutton(grid, 'push');
            obj.fileload1.FontSize = 36;
            obj.fileload1.Layout.Row = [8 9];
            % ...
        end
    end
end