[Bug] [`series-line`] When `areaStyle` is set to a gradient color and the mouse hovers over the line, the console reports: `Cannot read properties of undefined.`

[MrWhatHuang]

Version

5.6.0

Link to Minimal Reproduction

https://echarts.apache.org/examples/zh/editor.html?c=area-basic&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMmurGAJ4gCmRA5AMYCGYVA5sAE7m0A0J6AI2ABXaABNGXAOKMQRAGaMANnip9SsCWEZEA2rQCyMXrFoAVYVRO0A6lTHWzAC2HWAYhwjWAysx-jaAF0SAF91WHIcfCJUDQpqOgA3ZUtaUPDVTypCWF1-Yg10LR1cgA4AJgAGHlgATgBmcprayoBGZvqAFhrW8pae-vrq2FbGysDwuMoaUyUIaCtJ0kkqRm8KJRnYwvR6YCVOGPyd-JnaOYXJXmPCjCJqm41ye6WdjHKXx9JyD5HXwr2Bw461AOTyOx22whhWA8nkqjAn2hO0Bh1MHFYAgAFJURvVWrBygBWImwAD0IyJAFIAJRpZGkMJfDRQhmwWHwqiIv7MgH7NG0DHY3GjAnE0kUyq0-lskK84LQuU7JWMkjBEIAbiAA

Steps to Reproduce

in series-line
areaStyle set to a gradient color
mouse hovers over the line

Current Behavior

the console reports: Cannot read properties of undefined.

Expected Behavior

fix it

Environment

- OS:window 11
- Browser:Chrome 137.0.7151.119
- Framework: javascript

Any additional comments?

No response

[plainheart]

The color format rgb(0 131 255 / 15%) is not fully supported by ECharts, please use hex / rgb(r,g,b) / rgba(r,g,b,a) format instead.