Use subscripted assignment to create an array element.

I am try to plot data but I am getting an error. I don't know why. Any idea?

Sample Code

hold on
ax = gca;

ax.FontSize = 18;
ax.FontName = 'Times New Roman';

ax.XAxis.Color = 'k';
ax.YAxis(1).Color = 'k';
ax.YAxis(2).Color = 'k';

ax.XAxis.LineWidth = 2.5;
ax.YAxis(1).LineWidth = 2.5;
ax.YAxis(2).LineWidth = 2.5;

ax.XAxis.Limits = [0 1122];
ax.YAxis(1).Limits = [-5 25];
ax.YAxis(2).Limits = [-2 10];

ax.XAxis.TickLength = [0.02 0.035];
ax.YAxis(1).TickLength = [0.02 0.035];
ax.YAxis(2).TickLength = [0.02 0.035];


ax.XAxis.MinorTick = 'on';
ax.YAxis(1).MinorTick = 'on';
ax.YAxis(2).MinorTick= 'on';
 
ax.XAxis.MinorTickValues = 0:50:1122;
ax.YAxis(1).MinorTickValues = -5:1:25;
ax.YAxis(2).MinorTickValues = -2:1:10;

 hold on
ax.XAxis.MajorTickChild.LineWidth = 1.5;
ax.XAxis.MinorTickChild.LineWidth = 1.5;

ax.YAxis(1).MajorTickChild.LineWidth = 1.5;
ax.YAxis(1).MinorTickChild.LineWidth = 1.5;
 
ax.YAxis(2).MajorTickChild.LineWidth = 1.5;
ax.YAxis(2).MinorTickChild.LineWidth = 1.5;

Error

Property assignment is not allowed when the object is empty.  Use subscripted assignment to create an array element.

Error in FanHeaterSensorResponseTimeWithCurrentCurve (line 72)
ax.XAxis.MinorTickChild.LineWidth = 1.5;

[VBBV]

You can use the axes handle inside hold on to continue . Using hold on without axes handle creates a new figure with empty axes data.

hold(ax,'on')  % use axes handle 
ax.XAxis.MajorTickChild.LineWidth = 1.5;
ax.XAxis.MinorTickChild.LineWidth = 1.5;
ax.YAxis(1).MajorTickChild.LineWidth = 1.5;
ax.YAxis(1).MinorTickChild.LineWidth = 1.5;
 
ax.YAxis(2).MajorTickChild.LineWidth = 1.5;
ax.YAxis(2).MinorTickChild.LineWidth = 1.5;

or try

ax.XAxis.MajorTickChild.LineWidth = 1.5;
ax.XAxis.MinorTickChild.LineWidth = 1.5;
ax.YAxis(1).MajorTickChild.LineWidth = 1.5;
ax.YAxis(1).MinorTickChild.LineWidth = 1.5;
 
ax.YAxis(2).MajorTickChild.LineWidth = 1.5;
ax.YAxis(2).MinorTickChild.LineWidth = 1.5;