You can trim a Webm the same way you trim a MP4 video. However, you can cut multiple sections and stich the trimmed videos together with our product.
That said, if RealTrimmer shuts down when you load the converted MP4 file, it is most likely that the file didn't convert properly or RealPlayer is not installed properly. Can you please reinstall RealPlayer and use RealTrimmer and load your Webm file and let us know if it still crashes. If it does, use RealConverter and convert it to MP4 again and try with the newly converted MP4.